The Handy Calendar

What day of the week is it? For calendar calculating like savant syndrome.

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Procedures

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 (except that if Y = 0, Y' = 9999). If M ≥ 3, Y' = Y.

By implementing the following process, if the position that you finally come to is 0, 1, …, 6, Wed, Thu,…, Tue is your desired day. However, for integers a,b (b≠0), you divide a by b and write the remainder a%b.

Supplement:
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 “Pro”:

Step 1
Hold on 0.
Step 2
Shift the held position to n-after that n corresponds to M (Table 1) and count M.
Step 3
Shift the held position to (D-1)%7-after and count D.
Step 4
Shift the held position to 0-after and count maximum factor of 400 within a range equal to Y'or less.
Step 5
Shift the position to be held to 1-after every time when 300 are added as long as the total number does not exceed Y'.
Step 6
Shift the position to be held to 3-after every time when 200 are added as long as the total number does not exceed Y'.
Step 7
Shift the position to be held to 5-after every time when 100 are added as long as the total number does not exceed Y'.
Step 8
Shift the position to be held to 2-after every time when 80 are added as long as the total number does not exceed Y'.
Step 9
Shift the position to be held to 5-after every time when 60 are added as long as the total number does not exceed Y'.
Step 10
Shift the position to be held to 1-after every time when 40 are added as long as the total number does not exceed Y'.
Step 11
Shift the position to be held to 4-after every time when 20 are added as long as the total number does not exceed Y'.
Step 12
Shift the position to be held to 3-after every time when 8 are added as long as the total number does not exceed Y'.
Step 13
Shift the position to be held to 5-after every time when 4 are added as long as the total number does not exceed Y'.
Step 14
Shift the position to be held to 1-after every time when 1 is added as long as the total number does not exceed Y'.

(End Process)

     Table 1
1 2 3 4 5 6 7 8 9 10 11 12
5 1 0 3 5 1 3 6 2 4 0 2