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
Determination of the day of the week where is the day of the week (counting upwards from 1 on Sunday instead of 0 in Gauss's version); and , , and are numbers depending on the day, month, century and year which are tabulated in the "Complete table: Julian and Gregorian calendars" above. Note that the numbers tabulated for y can also be described by the following equation: Where So, for example, if you want to find 'y' for the year 1987: In Zeller’s algorithm, the months are numbered from 3 for March to 14 for February. The year is assumed to begin in March; this means, for