Question

when do we observe a leap year

Answer 1

we observe a leap year after every 4 year when February has 29 days Next leap year will be on 2020.

Answer 2

A leap year is observed every four years. In the leap year, an 'extra day' is added to the month of February.

EXPLANATION:

Every four years, February has 29 days and the year is called a leap year. A 'day' is added to the month of 'February' every four years as a 'corrective measure'.

This is because 'earth' doesn't  orbit the 'sun' or complete a revolution in 365 days. If a day is not added, we will lose 6 hours every calendar year.