Leap Years
A leap year has 366 days, with February 29 added to keep the calendar in step with the seasons. 2028 is the next leap year.
Check a year
The leap year rules
A year is a leap year if it can be divided by 4, except for years divisible by 100, which are not leap years unless they can also be divided by 400. So 2024 and 2028 are leap years, 1900 and 2100 are not, and 2000 was.
The extra day keeps the calendar in line with the solar year of about 365.2422 days. Without it, the seasons would drift by about a day every four years.