Divide the number of the given year by 4, neglecting the remainders, and add the quotient to the given number. Divide this amount by 7, and if the remainder be less than three, take it from 3; but if it be 3 or more than 3, take it from 10 and the remainder will be the number of the letter calling A, 1; B, 2; C, 3, etc.
By this rule the dominical letter is found from the commencement of the era to October 5th, 1582. O. S. From October 15th, 1582, till the year 1700, take the remainder as found by the rule from 6, if it be less than 6, but if the remainder be 6, take it from 13, and so on according to instructions given in the table on 49th page. It should be understood here, that in leap-years the letter found by the preceding rule will be the dominical letter for that part of the year that follows the 29th of February, while the letter which follows it will be the one for January and February.
To find the dominical letter for 1365, we have 1365 ÷ 4 = 341 +; 1365 + 341 = 1706; 1706 ÷ 7 = 243, remainder 5. Then 10 - 5 = 5; therefore E being the fifth letter is the dominical letter for 1365.
To find the dominical letter for 1620, we have 1620 ÷ 4 = 405; 1620 + 405 = 2025; 2025 ÷ 7 = 289, remainder 2. Then 6 - 2 = 4; therefore, D and E are the dominical letters for 1620; E for January and February, and D for the rest of the year. The process of finding the dominical letter is very simple and easily understood, if we observe the following order:
4th. Take the remainder from 3 or 10, from the commencement of the era to October 5th, 1582. From October 15th, 1582 to 1700, from 6 or 13. From 1700 to 1800, from 7, and so on. See table on 49th page.
We divide by 4 because the intercalary period is four years; and as every fourth year contains the divisor 4 once more than any of the three preceding years, so there is one more added to the fourth year than there is to any of the three preceding years; and as every year consists of 52 weeks and one day, this additional year gives an additional day to the remainder after dividing by 7. For example, the year
1 of the era consists of 52 w. 1 d. 2 years consist of 104 w. 2 d. 3 years consist of 156 w. 3 d. (4 ÷ 4) + 4 = 5 years consist of 260 w. 5 d.