Until the 16th century, 25 December coincided with 29 Koiak of the Coptic calendar. However, upon the introduction of the Gregorian calendar in 1582, 25 December shifted 10 days earlier in comparison with the Julian and Coptic calendars. Furthermore, the Gregorian calendar drops 3 leap days every 400 years to closely approximate the length of a solar year. As a result, the Coptic Christmas advances a day each time the Gregorian calendar drops a leap day (years AD 1700, 1800, and 1900). This is the reason why Old-Calendarists (using the Julian and Coptic calendars) presently celebrate Christmas on 7 January, 13 days after the New-Calendarists (using the Gregorian calendar), who celebrate Christmas on 25 December. From AD 2101, the Coptic Christmas will be on the Gregorian date of 8 January.