RicOlie_2 said:

February 24:

A.D. 1582: Pope Gregory XIII announces the new calendar (the Gregorian and modern calendar) as a replacement of the less accurate Julian calendar.

The question remains, was this on February 24 of the Julian Calendar, or on February 24 of the Gregorian Calendar? :P

Actually, the correct answer is of course this announcement was made under the Julian Calendar, as Gregorian was not implemented anywhere until October of 1582, and then only in four countries.