In 46 B.C., Roman ruler Julius Caesar put a new calendar into effect, which came to be known as the Julian Calendar. The Romans thought that the year was 365.5 days long, so they made an ordinary year 365 days and added an extra day every fourth year, or leap year.
But by the year 730 A.D., it was known that the year was actually 11 minutes shorter than the Romans of Julius Caesar’s time thought it was. This mistake made the calendar wrong by 11 minutes each year, or one day wrong in every 128 years. By 1582, the calendar was ten days out of line with the seasons.
So in that year, Pope Gregory XIII ordered a new calendar placed into effect, which we now call the Gregorian Calendar. To keep the calendar in line with the seasons, it was decided that the first year of each century would be a leap year only when that year could be divided by 400. Therefore, 1200 and 1600 were leap years, but 1800 and 1900 were not. This change eliminated the 11-minute-per-year error in the calendar.
To bring the calendar immediately back into line with the seasons, Gregory ordered that ten days be dropped from the year 1582. People who went to bed on the night of Oct. 5 of that year woke up on Oct.15!
Some branches of the Eastern or Orthodox churches still figure their holidays according to the Julian Calendar, which is now 13 days different from ours!