February is the only month in the year to have 28 days. There are seven months that have 31 days and four months that have 30 days.
The calendar year is aligned with the Earth's rotation around the sun, which takes approximately 365.25 days. The modern calendar is the Gregorian calendar, which only has 365 days in each year. To keep the calendar aligned with the sun, an extra day is added to February every four years. A year with 29 days in February is known as a leap year because the calendar "leaps" ahead a day to correct for the difference.