If you would like to know a date on the Western (Gregorian) calendar from October 15, 1582 onward, you use Y for the last four digits of the year, M for the month, and D for the day.
If M < 3, Y’ = Y - 1 (However, when Y = 0, Y’ = 9999), if M ≥ 3, then for Y’ = Y, use C for the 2 digits above Y’, and N for the 2 digits below Y’. (For example, if Y’ = 1234, C = 12 and N = 34.)
By implementing the following process, if the position that you finally come to is 0, 1, …, 6, Sun, Mon,…, Sat is your desired day. However, for integers a,b (b≠0), you divide a by b and write the remainder a%b.
On the joints of the index, middle, and ring fingers on your left hand, place the following numbers in the 7 marked out positions. From the tip of your index finger: 0,1,2. From the tip of your middle finger: 3,4,5. On the tip of your ring finger: 6. For integers a,b, 0≤a<7, and 0≤b<7, the position b-after of a indicates position (a+b)%7. See also n-after.
A method to guess the day of the week of a specific date by calculation using fingers "Mutant (tentative)”:
Hold on 0.
Shift the hold position to n-after that n corresponds to M (Table 1).
Shift the hold position to (D%7)-after.
Shift the hold position to n-after that n corresponds to C%4 (Table 2).
Use n' for the maximum multiples of 4 below N, and shift the hold position to n-after that n corresponds to n' (Table 3).
Shift the hold position to (N%4)-after.