Letters A through H used to mark all days of the year
when the year was divided into nundinæ, cycles of eight days.
Today all days of the year are marked with a letter A though G for each day of the week.
The dominical letter of the year
is determined by which day of the week the first day of January falls on.
This helps with calculations for the liturgical year, to ensure, for example,
that Ash Wednesday falls on a Wednesday, and Easter Sunday falls on a Sunday.
See also in The book of science:
Readings in wikipedia: