The 12 months in the table above don’t all have the same number of days, which trips people up more often than you’d expect. Seven months — January, March, May, July, August, October, and December — have 31 days. Four months — April, June, September, and November — have 30. February is the odd one out with just 28 days most years, and 29 during leap years.
Generations of schoolchildren have relied on a simple rhyme to remember the pattern: “Thirty days hath September, April, June, and November; all the rest have thirty-one, except February alone, which has twenty-eight days clear, and twenty-nine in each leap year.” It’s a handy shortcut precisely because the lengths aren’t evenly distributed — a side effect of centuries of calendar reforms rather than any deliberate design.
The unevenness traces back to the Roman calendar, which was adjusted and re-adjusted over time — including Julius Caesar’s reform that produced the Julian calendar — before evolving into the Gregorian calendar most of the world uses today. Leap years exist to correct for the fact that Earth’s orbit around the Sun takes about 365.25 days rather than an even 365, so an extra day is added to February roughly every four years to keep the calendar aligned with the seasons.