who came up with the idea of leap year?

Answer 1

Pope Gregory XIII in 1582


is the most widely used calendar in the world. A modification of the Julian calendar, it was first proposed by the Calabrian doctor Aloysius Lilius, and was decreed by Pope Gregory XIII, for whom it was named, on 24 February 1582 via the papal bull Inter gravissimas. Years in the calendar are numbered from the traditional birth year of Jesus, which was labeled the "anno Domini" (AD) era[1], and is sometimes labeled the "common era" (CE).

The Gregorian Calendar was devised both because the lunar calendar had grown conspicuously wrong, and the mean Julian Calendar year is slightly too long, so that the vernal equinox slowly drifts backwards through Julian calendar years. This caused problems in computing the date of Easter.
Tomb of Pope Gregory XIII in St. Peter's Basilica.
Tomb of Pope Gregory XIII in St. Peter's Basilica.

The Gregorian calendar system dealt with these problems by dropping a certain number of days to bring the calendar back into synchronization with the seasons, and then slightly shortening the average number of days in a calendar year, by omitting three Julian leap-days every 400 years. The days omitted are in the century years, i.e.: 29 February 1700, 1800, 1900, 2100, 2200, 2300, 2500, 2600, 2700, 2900 etc.
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Answer 2

Try this link out. It's got a detailed explanation.