The calendar year has 365 days, but the Earth only makes up about 365.25 days in one year. This is because it takes the Earth about 0.25 days to complete its orbit around the Sun. This difference of 0.25 days accumulates over time and causes a slight shift in when we should celebrate the new year each year.
The leap day is an extra day added to February every 4 years which makes some February months have 29 days. Typically, it occurs in years evenly divisible by 4, but not in any other year. It was inserted in the calendar over 2000 years ago to keep our months aligned with the cycles of the sun.
Leap year is an infrequent occurrence when an extra day is added to February. Common dates for leap years are February 29th through March 1st.
Leap years were first added to the Julian calendar by Julius Caesar, in 46 BCE, under the advice of the Alexandrian astronomer named Sosigenes.