[Nateman]

It is every 4 years, except for century turnover. Every 400 years that rule is ignored so 2000 was a leap year. This makes the Gregorian calendar 365.2425 days long which is very close to the actual time for Earth’s orbit period .

[Nateman]

1788 and 1896 were leap years. If it divides by 4 evenly it is a leap year, century years the exception. The world only went to the Gregorian calendar relatively recently. Russia , for example, waited many many years before adapting the Gregorian calendar.