The standard calendar that is used all over the world has a common year and a leap year. A common year has 365 days and a leap year 366 days, the extra day is the 29th of February. Leap year occurs once in 4 years, this helps to synchronize the calendar year with the solar year. It takes the Earth 365 1/4 days to complete its orbit around the sun, this clarifies why we have one extra day in four years.
The solar year is actually 365 days 5 hours 48 minutes and 45 seconds. To correct this discrepancy, 1 day is removed in every 100 years. The leap year is omitted 3 times in 400 years. That's why years like 1700, 1800 and 2100 are not leap years.
