How Many Months Have 28 Days (Calendar Math & Trivia)

The Logic & History of 28 Days in Calendar Months

The question "How many months have 28 days?" is one of the world's most famous calendar riddles. People instinctively answer "One" (thinking of February). But mathematically, all 12 months have at least 28 days:

• Seven months have 31 days (which includes 28 days + 3 days).
• Four months have 30 days (which includes 28 days + 2 days).
• February has 28 days in common years, or 29 days in leap years.

Why Does February Have Fewer Days?

In the 8th century BCE under Roman King Numa Pompilius, the Roman calendar had 10 months plus winter months. The Romans considered even numbers unlucky, so months had 29 or 31 days. To fit a 355-day lunar year, February was assigned 28 days as the end-of-year purification month (Februa). When Julius Caesar reformed the calendar in 45 BCE into 365 days, February was kept at 28 days with a leap day every 4 years.