The remaining dates of 2024 will take a big leap. They’ll jump over a day of the week. That’s because this is leap year, and today is leap day — extending the year from 365 days to 366.

Leap years are needed to keep the calendar in sync with the seasons. Without it, the equinoxes and solstices would slide across the calendar. So after many centuries, the spring equinox would happen in February, with the winter solstice backing into November. And over the ages, the shift would grow even larger.

The modern calendar is based on one instituted by Julius Caesar. It had a 365-day year, with a leap day added to every fourth year, for an average of 365 and a quarter days per year. But the true year is about 11 minutes shorter than that average, so the calendar drifted out of alignment with the seasons.

To fix that problem, in 1582 Pope Gregory XIII instituted an update. Under this calendar, three leap days are dropped off for every 400 years. With this change, the difference between the calendar year and the astronomical year adds up to one day every 3300 years.

The name “leap” year comes from the fact that the extra day causes succeeding dates to leap over a day of the week. For example, Christmas fell on a Monday last year. Without the leap day, this year it would fall on a Tuesday. But with the extra day, it will “leap” over Tuesday and fall on Wednesday.

Script by Damond Benningfield