Earth completes one revolution around the Sun for every 365.242199 times it rotates on its axis. This figure is, unfortunately, not evenly divisible, which has historically caused difficulties for the creators of various calendars.
A 360-day year would permit the use of a simple calendar of 12 30-day months. The extra five days require that a 12-month calendar have months of differing lengths. Even then, the unaccounted-for remainder of approximately .25 days forced the inclusion of quadrennial leap days in the Julian calendar. Even adding a leap day failed to stabilize the calendar because the remainder was actually slightly less than .25 days. This was addressed by the Gregorian calendar, which omits leap days in centenary years unless the date is divisible by both 4 and 400.