Determination of the day of the week not a Gregorian leap year, (but 19 in the Julian column indicates that it "is" a Julian leap year, as are all Julian "x"00 years). 20 indicates that 2000 is a leap year. Use Jan and Feb only in leap years. For determination of the day of the week (1 January 2000, Saturday) For determination of the dominical letter (2000, BA) For determination of the doomsday (2000, Tuesday) Note that the date (and hence the day of the week) in the Revised Julian and Gregorian calendars is the same from 14 October 1923 to 28 February AD 2800 inclusive and
Determination of the day of the week ISO 8601. The day designated with 7 may also be counted as "0", by applying the arithmetic modulo 7, which calculates the remainder of a number after division by 7. Thus, the number 7 is treated as 0, 8 as 1, 9 as 2, 18 as 4 and so on. If Sunday is counted as day 1, then 7 days later (i.e. day 8) is also a Sunday, and day 18 is the same as day 4, which is a Wednesday since this falls three days after Sunday. The basic approach of nearly all of the methods to calculate the