Fun related fact, many dates -- for example, birth dates on headstones -- of people born in countries observing Eastern Orthodox Christianity will have the prefix O.S. (old style) to disambiguate dates using the original Julian and either the Georgian or Revised Julian calendars which were in effect at that person's death.

A primary reason for the Catholic Church changing the calendar was to bring the date of Easter back into conformance with the decree of the Council of Nicaea on the topic. There was a controversy in the early Church on this, there were even miracles worked concerning the correct date -- see Dom Gueranger's "Liturgical Year" for some of the accounts.