Every month except February contains either 30 or 31 days in the modern calendar.

The longer version

The modern civil calendar hands out days so that most months run either 30 or 31, alternating in a pattern that people still trace across their knuckles. February alone breaks it, holding 28 days or 29 when the year carries an extra one. No astronomical reason requires the split; it is purely administrative.

The irregularity traces back to Roman practice, where days were pared from the last winter month and donated to others. Because the spread is uneven, quarters differ in length and monthly comparisons can mislead, so analysts often divide totals by the number of days before drawing conclusions about activity. The system survives because changing it would be far more disruptive than living with the awkwardness.

Where this entry sits

These facts wander through clocks, calendars, time zones, and the long human effort to measure passing hours. You will find leap years, odd month lengths, ancient sundials, and the reasons modern timekeeping works the way it does. The full list sits on the Time and Calendars page, where every entry is listed in order.