250
It was not invented but selected it started on May 1866.
1, May 1 is May Day and there is only one May 1 every year.
Year 0 began at 00:00:00 hrs and that morning was a Sunday.
Stricly speaking there a re 365 and a quarter, but we only have 365, then add up the quarters ever four years and have an extra day, making 366 days. Those are leap years and I think the next one is...