Which of these years in Gregorian calendar is a leap year with 366 days?

A leap year is a calendar year that contains 366 days instead of 365 days. In the Gregorian calendar, this extra day is added in the month of February as the 29th day of that month.

In order to be a leap year, the year must be evenly divisible by 4 with no remainders, but it cannot be a multiple of 100 unless it is also a multiple of 400. So year 1896 was a leap year, but year 1900 was not. Year 2000 was a leap year even though 2000 is divisible by 100 because it is also divisible by 400.

When 1 day is added to a year every 4 years, the average length of 1 year is defined as exactly 365.2500 days. However, the actual time it takes for the Earth to complete one orbit around the Sun is 365 days 48 minutes and 46 seconds, or 365.2422 days. Therefore, in 400 years, this results in overestimation of 3.12 days (400 x 0.0078). This is why 3 days must be taken out during a 400-year span. This is achieved by taking away one leap year every century for 3 consecutive centuries, but not in the next century.