Ten days were skipped

In the Catholic world in 1582, Thursday 4 October was followed by Friday 15 October. Ten dates were removed to realign the calendar with the Sun.

The Julian calendar had been slipping against the seasons by about one day every 128 years, because its leap-year rule overcorrects. By the sixteenth century the spring equinox, used to place Easter, was ten days early relative to the Council of Nicaea. Gregory XIII's bull Inter gravissimas cut ten days out of October 1582 and changed the leap-year rule: century years are not leap years unless divisible by 400.

Rome, Spain, Portugal, and Poland jumped. France followed in December. Britain and its colonies waited until 1752 and then skipped eleven days, 3 September through 13 September. Russia did not adopt the Gregorian calendar until the twentieth century, which is why the October Revolution fell in November by the calendar everyone else was using.