Dominical letters are letters A, B, C, D, E, F and G assigned to days in a cycle of seven with the letter A always set against 1 January as an aid for finding the day of the week of a given calendar date and in calculating Easter.

A common year is assigned a single dominical letter, indicating which letter is Sunday (hence the name, from Latin dominica for Sunday). Thus, 2011 is B, indicating that B days are Sunday. Leap years are given two letters, the first indicating the dominical letter for January 1 - February 28 (or February 24, see below), the second indicating the dominical letter for the rest of the year.

In leap years, the leap day may or may not have a dominical letter. In the original 1582 Catholic version, it did, but in the 1752 Anglican version it did not. The Catholic version caused February to have 29 days by doubling the sixth day before 1 March, inclusive, because 24 February in a common year is marked "duplex", thus both halves of the doubled day had a dominical letter of F. The Anglican version added a day to February that did not exist in common years, 29 February, thus it did not have a dominical letter of its own.

Dominical is a beach-front town in the province of Puntarenas in Costa Rica approximately 45 km south of Quepos. It is famous for large, year-round waves and is well-known by most surfers in Costa Rica. The town began as a small fishing village but has since grown into a tourist destination, most notably for surfers.

Access from the capital, San José can be via San Isidro de El General to the east or down the coast from Quepos. The area between Quepos and Dominical is dotted with African oil palm plantations producing palm oil. Farther south are the villages of Uvita, Ojochal, Coronado on the Costanera Highway.

Coordinates: 9°15′N 83°52′W / 9.25°N 83.867°W / 9.25; -83.867