The Julian calendar was a complete reform on the lunar calendar that existed before in ancient Rome. In the year 46 BC, and at the suggestion of Julius Caesar, the Alexandrian sage Sosígenes changed the existing calendar year establishing a new one with 365 days, and an additional day every four years. Every month was Set 30 and 31 days, except for February which was going to have 28 or 29 depending on whether or not the year was a leap year. The start of the year was moved from March 1 to January 1.

Some countries did not adopt this system. Thus England, and later their American colonies, held onto the first day of the year on March 25, coinciding with the spring equinox.

In 1582, Pope Gregory XIII reformed the Julian calendar. Over the centuries, the date for the spring equinox became variable, as established in the Council of Nicea.
For the calendar to coincide with the solar seasons, it was established there would be no leap years at the end of each century that were not divisible by 400. Apart from this fundamental reform of the Julian calendar, an error had been accumulated (10 days) – Pope Gregory removed 10 days, going directly to the 4-15 October 1582. This new calendar is known as the Gregorian calendar, which is currently being used, but is not suitable for all nations. Indeed, some Countries took centuries to use. That is the case of England and its colonies, which began using the new calendar in 1752. Sweden adopted it a year later. Others took much more time, as Turkey did not accept it until 1917 and Russia until 1918. Christopher Clavius was the primary architect of the Gregorian calendar. He, along with its partners, established a set of rules for determining the date of Easter.
They chose to begin, March 21 as the time when we passed the spring equinox and a letter (the dominical) and a number, (the EPACT age of the moon on January 1).

Sunday gives us the letters of the day, weeks and the dates that fall on Sundays. Each day in the week is associated with a letter ; there are 7 letters, one for each day. For example, A corresponds to January 1, B 2, C 3 and so on until the G corresponds to January 7.
Then the sequence repeats itself: A for January 8, B for 9, etc ... For example in 2000, the first Sunday is January 2 - therefore B is the dominical letter for all of 2000. This would be true if the year is normal, but 2000 is a leap year, and leap years have two dominical letters, in this case for year 2000 – BA is the dominical letter.