Dominical letterThe days of the year are sometimes designated letters A, B, C, D, E, F and G in a cycle of 7 as an aid for finding the day of week of a given calendar date and in calculating Easter. These letters are known as dominical Letters.
A common year has a dominical letter, which is simply the dominical letter of its first Sunday. For example 2003 has 5 January as its first Sunday so has Dominical letter E.
In leap years, the leap day has no dominical letter. This ensures that each date has the same dominical letter every year, but causes the days of the weeks of the dominical letters to change within a leap year. Hence leap years have two dominical letters. The first for January and February and the second for March to December. The second dominical letter is the dominical letter of the first Sunday of October (which is the same as for January in a common year). The year 2004 has Dominical letters DC.
A common year starting on Sunday B common year starting on Saturday C common year starting on Friday D common year starting on Thursday E common year starting on Wednesday F common year starting on Tuesday G common year starting on Monday
AG leap year starting on Sunday BA leap year starting on Saturday CB leap year starting on Friday DC leap year starting on Thursday ED leap year starting on Wednesday FE leap year starting on Tuesday GF leap year starting on Monday
- Catholic Encyclopedia