The Julian calendar has a regular year of 365 days divided into 12 months with a leap day added to February every four years. The Julian year is, therefore, on average 365.25 days long. The motivation for most calendars is to fix the number of days between return of the cycle of seasons (from Spring equinox to the next Spring equinox, for example), so that the calendar could be used as an aid to planting and other season-related activities. The cycle of seasons (tropical year) had been known since ancient times to be about 365 and 1/4 days long.
The approximation of 365¼ days for the tropical year had been known for a long time but was not used directly since ancient calendars were not solar (except in Egypt). It was the mean length of the year in the octaeteris, a cycle of 8 lunar years popularized by Cleostratus (and also commonly attributed to Eudoxus) which was used in some early Greek calendars, notably in Athens. The same value was the basis of the 76 years cycle devised by Callippus (a student under Eudoxus) to improve the Metonic cycle. The length of the year in the cycle of Meton was less accurate: about 30 minutes too long (1 day in 48 years).