The Coptic calendar is used by the Coptic Orthodox Church. It divides the year into 13 months: 12 months of 30 days each plus an intercalary month of either five or six days, depending on whether the year is a leap year.
The Coptic year begins on the Feast of Neyrouz, the first day of the month called Tout, which is equivalent to September 11 in the Gregorian calendar, except before a Gregorian leap year when it's September 12.
Every fourth year is a leap year without exception, like in the Julian calendar, so the above mentioned new year dates apply only between 1900 and 2099 A.D inclusive in the Gregorian Calendar. In the Julian Calendar, the new year is always August 29, except before a Julian leap year when it's August 30. Easter is reckoned by the Julian Calendar in the Old Calendarist way.