Real Dictionary

dominical letter in American English
any of the first seven letters in the alphabet as used in church calendars to indicate Sundays: the letters are assigned to the first seven days of January, and the letter falling to Sunday is the arbitrary symbol for Sunday the rest of the year
noun: any one of the letters from A to G used in church calendars to mark the Sundays throughout any particular year, serving primarily to aid in determining the date of Easter

dominical letter in British English
noun: any one of the letters A to G as used to denote Sundays in a given year in order to determine the church calendar