Dominical letter

The 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.

Examples include

The dominical letter of a year determines the days of week in its calendar