Why Do Months Have Different Lengths?

January has 31 days, February has 28 (or 29), April has 30. There is no mathematical pattern, no astronomical reason, and no logical system behind it. The answer involves an ancient Roman king, two power-hungry emperors, and a month that has been getting shortchanged for over 2,000 years.

May 2, 2026 · 11 min read · Gabriel

Bar chart comparing the number of days in each month of the year

Table of Contents

The calendar makes no sense

Look at the months of the year and try to find a pattern. January: 31 days. February: 28 (or 29). March: 31. April: 30. May: 31. June: 30. July: 31. August: 31. September: 30. October: 31. November: 30. December: 31.

Two months in a row with 31 days (July and August). One month with only 28. No clean alternation between 30 and 31. And the shortest month falls in winter in the Northern Hemisphere, when dark days already feel endless. None of it follows any discernible logic.

There is a reason for this mess, but it is not astronomical and it is not mathematical. It is political. The lengths of our months are the fossilized remains of decisions made by Roman kings, priests, and emperors over a span of about 700 years. To understand why February has 28 days while its neighbors have 31, you need to go back to the very beginning of the Roman calendar.

Romulus and the 10-month year

According to Roman tradition, the original Roman calendar was created by Romulus, the legendary founder of Rome, around 753 BC. This calendar had only 10 months and began in March — named after Mars, the god of war, because spring was when military campaigns resumed after winter. The 10 months were:

Martius (March) — 31 days
Aprilis (April) — 30 days
Maius (May) — 31 days
Iunius (June) — 30 days
Quintilis — 31 days (later renamed July)
Sextilis — 30 days (later renamed August)
September — 30 days
October — 31 days
November — 30 days
December — 30 days

Notice the names: Quintilis means "fifth," Sextilis means "sixth," September means "seventh," October means "eighth," November means "ninth," and December means "tenth." These numerical names still make sense in a calendar that starts in March. They only became absurd when January and February were later added to the beginning, pushing each month two positions forward.

This 10-month calendar totaled only 304 days. The remaining 61 days of winter were simply not counted — they belonged to no month at all. Romans essentially treated deep winter as a dead period outside the calendar, a nameless void between December and the next March. Agriculture had stopped, military campaigns were suspended, and civic life slowed to a crawl. Why bother counting days that did not matter?

Numa fills the gap: January and February

The uncounted winter gap was a problem. As Rome grew from a small settlement into a regional power, business and governance could not simply stop for two months. Around 713 BC, King Numa Pompilius, Rome's second king, reformed the calendar by adding two new months to fill the void: Ianuarius (January), named after Janus, the two-faced god of doorways and beginnings, and Februarius (February), named after februa, the Roman purification rituals held at that time of year.

Numa placed these months at the end of the year, after December. The year still began in March. But now the calendar covered all 12 lunar cycles, totaling approximately 355 days — close to 12 lunar months of 29.5 days each.

The Romans considered even numbers unlucky. Numa therefore adjusted the months so that most had 29 or 31 days, avoiding 30 wherever possible. January received 29 days and February received 28. February's 28 days made it the only month with an even number of days — a distinction that was considered appropriate because February was the month of purification and the dead, where a touch of ill fortune was ritually acceptable.

To keep the calendar roughly aligned with the solar year of 365.25 days, a short intercalary month called Mercedonius (or Mensis Intercalaris) was inserted every two years or so, usually in the middle of February. This intercalary month was 27 or 28 days long, and the remaining days of February were tacked onto the end of it. The system was clumsy but functional — as long as the priests responsible for declaring intercalations did their job.

They did not. The pontifices (the priestly college in charge of the calendar) had enormous discretion over when to insert the extra month, and they routinely abused it. A pontifex could extend a year to keep a political ally in office longer, or shorten a year to cut short an opponent's term. By the first century BC, the calendar was a political weapon, hopelessly out of sync with the actual seasons. The spring planting festivals were falling in winter. Harvest celebrations occurred before the crops were in. The calendar had drifted by weeks, sometimes months.

Julius Caesar fixes everything (almost)

By 46 BC, the Roman calendar was so broken that January was falling in autumn. Julius Caesar, who had spent time in Egypt and seen the superior Egyptian solar calendar of 365 days, decided on a radical reform. He consulted the Alexandrian astronomer Sosigenes and together they designed what we now call the Julian calendar.

The changes were sweeping. The year would be 365 days long, with a leap day added every four years. Months would no longer follow the Moon at all — they would simply divide the solar year into 12 roughly equal parts. The intercalary month was abolished forever.

Caesar redistributed the days to give the months their new lengths. The pattern he established was relatively orderly: months alternated between 31 and 30 days, except for February, which kept its traditional 28 (29 in leap years). Starting from January:

January: 31, February: 28(29), March: 31, April: 30, May: 31, June: 30
Quintilis: 31, Sextilis: 30, September: 31, October: 30, November: 31, December: 30

This was a clean, logical system. Odd months had 31 days, even months had 30, and February was the sole exception. But this elegant arrangement would not survive the egos of Caesar's successors.

To bring the calendar back into alignment with the seasons, Caesar declared that the year 46 BC would be 445 days long. He inserted two extra intercalary months in addition to the regular Mercedonius, creating what the Romans called the annus confusionis — the "year of confusion." It was the longest year in recorded history, but it worked: on January 1, 45 BC, the Julian calendar began in proper alignment with the solar year, and the seasons fell where they belonged.

The starting month of the year was also officially moved from March to January — though this change had already been happening gradually for administrative reasons. This move is why September through December are now the 9th through 12th months despite their names meaning 7th through 10th.

Julius and Augustus steal their days

In 44 BC, Julius Caesar was assassinated. Shortly after, the Roman Senate honored him by renaming Quintilis (the fifth month in the old calendar, and Caesar's birth month) to Julius — July. July already had 31 days under Caesar's reform, so no change in length was needed.

The trouble began with Caesar's adopted heir, Augustus. In 8 BC, the Senate renamed Sextilis to Augustus — August — in honor of Emperor Augustus. But here arose a problem of vanity: in the original Julian calendar, August (Sextilis) had only 30 days while July had 31. Augustus could not accept that "his" month was shorter than his adoptive father's. So one day was taken from February (reducing it from 29 to 28 in common years) and added to August, giving it 31 days.

But now July and August both had 31 days, creating an awkward three-in-a-row with September (which also had 31). To fix the resulting imbalance, the lengths of the remaining months were swapped: September went from 31 to 30, October from 30 to 31, November from 31 to 30, and December from 30 to 31.

This is the origin of the modern month pattern. The clean alternation that Caesar designed was broken to satisfy one emperor's ego, and the ripple effects reshuffled the entire second half of the year. February, already the shortest month in the system, lost yet another day and has been stuck at 28 ever since.

Some historians dispute the "Augustus stole a day" story, arguing that the month lengths were set by Caesar himself and that the Augustan renaming did not involve any change in days. The debate has raged since at least the 13th century. But the traditional account explains the anomalies so neatly — why July and August both have 31, why February has only 28, why the alternation breaks in the middle of the year — that it has become the standard narrative.

February: the unlucky month

February has always been the oddest month in the calendar. In Numa's original system, it was the last month of the year — the tail end, the leftover, the month of purification before the new year began in March. When it was eventually moved to the second position (after January was placed at the start), it carried its shortness and its association with death and ritual cleansing with it.

In the oldest Roman religion, February was the month of Parentalia, a nine-day festival honoring the dead, and Lupercalia, an ancient purification festival involving animal sacrifice and ritualistic running. It was a liminal, transitional time — the boundary between the old year and the new. Giving it fewer days than other months was symbolically fitting: February was not a month for business or war, but for settling accounts with the past.

The placement of the leap day in February also has ancient roots. In the Roman system, the intercalary month was always inserted in the middle of February, between the 23rd and 24th. When Caesar replaced the intercalary month with a single leap day, he placed it in the same traditional spot: after February 23, as a doubled "sixth day before the Kalends of March" (bis sextus), which is where the word "bissextile" (leap year) comes from. This tradition persisted even as the leap day eventually migrated to February 29 in modern practice.

The knuckle trick and other mnemonics

Because the pattern of month lengths is so irregular, people have invented memory aids to keep track. The most famous is the knuckle mnemonic: make two fists and hold them side by side. Starting from the leftmost knuckle (January), count across. Knuckles are "high" months (31 days) and the valleys between them are "short" months (30 or 28). January (knuckle, 31), February (valley, 28/29), March (knuckle, 31), April (valley, 30), May (knuckle, 31), June (valley, 30), July (knuckle, 31). Then jump to the first knuckle of the right hand: August (knuckle, 31), September (valley, 30), October (knuckle, 31), November (valley, 30), December (knuckle, 31).

The knuckle trick works precisely because of the July-August anomaly: the "jump" from the last knuckle of one hand to the first knuckle of the other puts two knuckles (31-day months) in a row, perfectly matching the consecutive 31-day months of July and August.

The English-language rhyme "Thirty days hath September, April, June, and November" dates back to at least 1555, when it appeared in a manuscript by the English writer William Harrison. Variations of this verse exist in almost every European language, suggesting that the irregularity of month lengths has been a source of confusion for centuries.

Why nobody ever fixed it

Over the centuries, numerous reformers have proposed "rational" calendars with equal-length months. The most notable is the International Fixed Calendar, proposed by Moses B. Cotsworth in 1902 and championed by the Eastman Kodak Company. It divides the year into 13 months of exactly 28 days each (13 × 28 = 364), plus one extra "Year Day" that belongs to no month or week. The 13th month, called Sol, would be inserted between June and July. Every month would start on a Sunday and end on a Saturday. Every date would fall on the same day of the week, every year, forever.

The International Fixed Calendar was seriously considered by the League of Nations in the 1920s and 1930s. Kodak actually used it internally for decades (from 1928 to 1989) for financial planning. But it was never adopted internationally, for several reasons: 13 is a prime number, making quarterly division impossible; many cultures consider 13 unlucky; and religious groups objected to the "blank day" concept, which would break the continuity of the 7-day weekly cycle.

The World Calendar, proposed by Elisabeth Achelis in the 1930s, was another attempt: 12 months grouped into four identical quarters of 91 days (31 + 30 + 30), with one or two intercalary days. It reached the United Nations in 1954 but was blocked by religious objections to the blank-day problem.

The simple truth is that changing month lengths would require global coordination on a scale that makes metric conversion look easy. Every database, every contract, every piece of software that handles dates would need updating. Every birthday, anniversary, and holiday tied to a specific date would shift. And the benefits — slightly tidier accounting, slightly easier mental arithmetic — are far too small to justify the disruption. The Roman calendar's quirks are now embedded so deeply in our legal, financial, religious, and cultural systems that they are essentially permanent.

The months we have, and why

Here is the final accounting of how each month got its length:

January (31) — Set by Caesar's reform; named after Janus.
February (28/29) — The ancient leftover month, shortened further to give August an extra day; named after purification rites.
March (31) — The original first month; named after Mars.
April (30) — Origin of name debated (possibly from aperire, "to open," for the opening of flower buds).
May (31) — Named after Maia, goddess of growth.
June (30) — Named after Juno, goddess of marriage.
July (31) — Originally Quintilis ("fifth"); renamed for Julius Caesar in 44 BC.
August (31) — Originally Sextilis ("sixth"); renamed for Augustus and enlarged to 31 days.
September (30) — "Seventh month" by name, ninth by position; lost a day in the Augustan reshuffle.
October (31) — "Eighth month" by name, tenth by position; gained a day in the reshuffle.
November (30) — "Ninth month" by name, eleventh by position; lost a day.
December (31) — "Tenth month" by name, twelfth by position; gained a day.

The story of month lengths is, in the end, a story about human nature. We inherited a calendar shaped not by logic or science but by superstition (even numbers are unlucky), politics (priests manipulating intercalary months), vanity (Augustus demanding as many days as Julius), and inertia (nobody can agree on a replacement). The result is a system that makes no rational sense — and that every human on Earth uses without a second thought, every single day.