Wikipedia says this about it:
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 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.From ehow.com
The word Ganna is used interchangeably with the word Christmas to mean the birth of Christ (leddat). A common way to refer to the holiday is "ye ganna baal".