The Coptic calendar, also called the Alexandrian calendar, is used by the Coptic Orthodox Church and still used in Egypt. This calendar is based on the ancient Egyptian calendar. To avoid the calendar creep of the latter, a reform of the ancient Egyptian calendar was introduced at the time of Ptolemy III (Decree of Canopus, in 238 BC) which consisted of the intercalation of a sixth epagomenal day every fourth year. However, this reform was opposed by the Egyptian priests, and the idea was not adopted until 25 BC, when the Roman Emperor Augustus formally reformed the calendar of Egypt, keeping it forever synchronized with the newly introduced Julian calendar. To distinguish it from the Ancient Egyptian calendar, which remained in use by some astronomers until medieval times, this reformed calendar is known as the Coptic calendar. Its years and months coincide with those of the Ethiopian calendar but have different numbers and names.
The Coptic year is the extension of the ancient Egyptian civil year, retaining its subdivision into the three seasons, four months each. The three seasons are commemorated by special prayers in the Coptic Liturgy. This calendar is still in use all over Egypt by farmers to keep track of the various agricultural seasons. The Coptic calendar has 13 months, 12 of 30 days each and one intercalary month at the end of the year of 5 days in length, except in leap years when the month is 6 days. The year starts on 11 September in the Julian Calendar or on the 12th 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. Ignorant of the Egyptian language for the most part, the Arabs confused the Egyptian new year's celebrations, which the Egyptians called the feast of Ni-Yarouou (the feast of the rivers), with the Persian feast of Nowruz. The misnomer remains today, and the celebrations of the Egyptian new year on the first day of the month of Thout are known as the Neyrouz. Its celebration falls on the 1st day of the month of Thout, the first month of the Egyptian year, which for 1901 to 2098 usually coincides with 11 September, except before a Gregorian leap year when it is 12 September. Coptic years are counted from 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"). Note that A.M. abbreviation is also used for unrelated calendar eras (such as the Byzantine and Jewish calendar epochs) which start at the putative creation of the world; it then stands for Anno Mundi.