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The Coptic calendar has 13 months, 12 of 30 days each and an intercalary month at the end of the year of 5 or 6 days, depending whether the year is a leap year or not. The year starts on [[August 29|29 August]] in the Julian Calendar or on the 30th in the year before (Julian) Leap Years. The Coptic Leap Year follows the same rules as the Julian Calendar so that the extra month always has six days in the year before a Julian Leap Year.
The Feast of Neyrouz marks the first day of the Coptic year. Its celebration falls on the 1st day of the month of Thout, the first month of the Coptic year, which for AD 1901 to 2098 usually coincides with [[September 11|11 September]], except before a Gregorian leap year when it's [[September 12]]. Coptic years are counted from AD 284, the year [[Diocletian]] became Roman Emperor, whose reign was marked by tortures and mass executions of Christians, especially in Egypt. Hence, the Coptic year is identified by the abbreviation A.M. (for ''Anno Martyrum'' or "Year of the Martyrs"). The A.M. abbreviation is also used for the unrelated Jewish year (''Anno Mundi'').
Every fourth Coptic year is a leap year ''without exception'', as in the Julian calendar, so the above mentioned new year dates apply only between AD 1900 and 2099 inclusive in the Gregorian Calendar. In the Julian Calendar, the new year is ''always'' 29 August, except before a Julian leap year when it's [[August 30]]. [[Easter]] is reckoned by the Julian Calendar in the Old Calendarist way.
C is -4 from 1583 to 1699, -5 from 1700 to 1899, -6 from 1900 to 2199, -7 from 2200 to 2299 etc... As the Sunday following the PFM, Easter is one week after the PFM when the PFM happens to fall on a Sunday. One must work with the Julian calendar (C = +3) to find when Easter is celebrated by Orthodox churches.
At the Council of Nicaea, it became one duty of the Coptic Pope of Alexandria to determine the exact dates of Easter and to announce it to the rest of the Christian churches (see [http://www.copticcentre.com/copticsaints.html#Pope%20Demetrius%20The%20Vinedresser Pope Demetrius the Vinedresser, 3rd cent.]). This duty fell on this officate because of the erudition at Alexandria he could draw on. The precise rules to determine this are very involved, but Easter is usually the first Sunday after a full moon occurring no sooner than March 21, which was the actual date of the vernal equinox at the time of the First Council of Nicaea. Shortly before Julius Caesar reformed the calendar, the vernal equinox was occurring on the "nominal" date of March 25. This was abandoned at Nicaea, but the reason for the observed discrepancy was all but ignored (the actual tropical year is not quite equal to the Julian year of 365¼ days, so the date of the equinox keeps creeping back in the Julian calendar).
''See also'': [http://en.wikipedia.org/wiki/Computus Computus], [[Revised Julian Calendar]]