One of the first seven letters of the alphabet, "A" through "g," is assigned to each date in the calendar year (except Feb. 29) in rotation in the Prayer Book calendar (pp. 19-30). The letter "A" is assigned to Jan. 1, Jan. 8, Jan. 15, etc. The letter "b" is assigned to Jan. 2, Jan. 9, Jan. 16, etc. The Sunday Letter can be used to determine all the dates of the Sundays in a calendar year. For example, if Jan. 1 is a Sunday, "A" is the Sunday Letter. All dates in the table with the letter "A" will be Sundays. Leap Year presents the one exception to this rule. In a Leap Year, the letter corresponding to the date of the first Sunday in Jan. will be the Sunday Letter for the months of Jan. and Feb. However, the Sunday Letter for Mar. through Dec. in a Leap Year will be the letter preceding the Sunday Letter for Jan. and Feb. For example, if "g" is the Sunday Letter in Jan. and Feb. of a Leap Year, "f" will be the Sunday Letter in Mar. through Dec. of that Leap Year. This change results from the additional day (Feb. 29) in a Leap Year. The BCP provides a table for ready reference to the Sunday Letter of any year between A.D. 1900 and A.D. 2099 (pp. 880-881). It is possible to determine the date of Easter Day for any particular year in the calendar (pp. 21-22) by using the Golden Number and the Sunday Letter for that year (see Tables and Rules for Finding the Date of Easter Day, BCP, pp. 880-881). The Sunday Letter is also known as the Dominical Letter. Clergy and parishioners usually determine the dates of Sundays by consulting a printed calendar.
Glossary definitions provided courtesy of Church Publishing Incorporated, New York, NY,(All Rights reserved) from "An Episcopal Dictionary of the Church, A User Friendly Reference for Episcopalians," Don S. Armentrout and Robert Boak Slocum, editors.