Gregorian Calendar Reform

The Julian Calendar was established by Julius Caesar in 45 BC. It adds an
extra day every four years, to keep the calendar dates on track with the
position of the Sun. The actual year, however, is just a bit shorter than
this, so the Julian Calendar was getting ahead of the Sun's position.

By 1582, this difference amounted to about ten days. Pope Gregory XIII
established the calendar we use today, which is called the Gregorian
Calendar. There is still an extra day every four years, but only in every
fourth century year.

To bring the calendar back into alignment with the Sun, Pope Gregory
declared that the days between October 4 and October 15, 1582 should
be eliminated. That is why there is a gap in this month.