10 Days Missing in October 4, 1582.

A change was recommended by astronomers and made at the time that Gregory XIII was the pope, and so the corrected calendar with its "leap year" was called the Gregorian calendar. It began to function on Friday, the 5th of October, 1582. Friday the 5th was changed to Friday the 15th. So that particular month was ten days shorter, but the length of its weeks was not affected. This is due to the simple fact that the number of days in the month or in the year has nothing to do with the number of days in the week. Thus the weekly cycle was not affected in any way.

In the following diagram is the calendar change that took place in October, 1582.