What are the eleven missing days?

Answer 1

In the early days of the calendar, from Roman times, it was assumed that the length of the year was 365 and a quarter days, and so the every-four-years leap year was all that was needed to keep the world in sync with the sun. This was known as the Julian calendar. Over time, however, it was found that this was inaccurate, the length of the year is actually 365.243 days, slightly less than 365 and a quarter, and so the Gregorian calendar was created; the difference is slight, years ending in 00 are not leap years unless they end in 000, but the difference is there, and by the 1500s, the difference between the two calendars had grown to eleven days. The correction was applied by law, mandating that the calendar date suddenly jumped forward eleven days; this was the missing eleven days, and was extremely unpopular as many landlords still charged a full month's rent for an abbreviated month.

The Orthodox church still calculates its church holidays based on the Julian calendar, which is why their Easter and Christmas celebrations occur at different times than the Christian churches.