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) and M' = M + 12. If M ≥ 3, Y' = Y, and M' = M.

By implementing the following process, if the position that you finally come to is 0, 1, …, 6, Wed, Thu, …, Tue is your desired day.

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. However, for integers c,d (d≠0), you divide c by d and write the remainder c%d. See also n-after.

A method to guess the day of the week of a specific date by calculation using fingers “Beginner”:

Step 1
Hold on 0 position and count maximum factor of 400 within a range equal to Y'or less.
Step 2
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 3
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 4
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 5
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'.
Step 6
Shift the held position to 0-after and count 3.
Step 7
Shift the position to be held to 6-after every time when 5 are added as long as the total number does not exceed M'.
Step 8
Shift the position to be held to 5-after every time when 2 are added as long as the total number does not exceed M'.
Step 9
Shift the position to be held to 3-after every time when 1 is added as long as the total number does not exceed M'.
Step 10
Shift the held position to 0-after and count 1.
Step 11
Shift the position to be held to 0-after every time when 7 are added as long as the total number does not exceed D.
Step 12
Shift the position to be held to 1-after every time when 1 is added as long as the total number does not exceed D.

(End Process)