When did the actual concept of the leap year originate?
In 1582, on February 24th, was decided that the day after 'Thursday, October 4th, 1582' would be 'Friday, October 15th, 1582'. Details below...
In 1582, Pope Gregory XIII (13th), January 7th, 1502 - April 10th, 1585, Pope of the Catholic Church from May 13th, 1572 to his death in 1585, commissioned the adjustment of the Julian calendar, that was in use at that time, and which was already generating significant discrepancies over the astronomical year.
The new calendar was instituted when Pope Gregory decreed, by the papal bull Inter gravissimas of February 24th, 1582, that the day after Thursday, October 4th, 1582 would be not Friday, October 5th, but Friday, October 15th, 1582.