Pope Gregory revised the calendar in 1552 to realign the equinox dates. Older calendars had used, for example, a year length of 365 days instead of a more accurate 365.25 days (but see below). So in 1552 10 days were omitted from the year (there were additional revisions too). The protestant countries avoided this Papal correction until 1752 and had to omit 11 days because of a leap year issue in 1600. For a wonderful description of the Gregorian calendar and calendars in general, see this article by Professor Albert Van Helden of Rice University. Or see Wolfram Research for another technical description of the issues.

The length of the year is about 365 days, 5 hrs. 48 min. 46 sec. (365.2422 days), or about 365 1/4 days. One must take care with such numbers, the period of the earth's rotation around the sun is changing a bit as the years pass.

The Gregorian calendar corrects for the non-integral number of days in the year first by adding a 'leap-day' every four years. This correction makes the average year 365.25 days long. If that was the only correction our years would be one day too long every 128 years. We make a further correction by making only century years divisible by 400 leap years (so 2000 was a leap year while 1700, 1800 and 1900 were not). A little calculation shows that these corrections make the average year 365.2425 days long instead of the more accurate 365.2422 days. But with these corrections the calendar will be only one day off in about 3333 years.