In March 1887, Lewis Carroll devised a method for mentally computing the day of the week for any given date. Carroll could carry
out these cmputations in his head in about 20 seconds.
Take the given date in 4 portions, namely the number of centuries, the number of years over, the month, the day of the month.
Compute the following 4 items, adding each, whenever found, to the total of the previous items. When an item or total exceeds 7,
divide by 7, and keep the remainder only.
The Century-Item: Divide by 4, take overplus from 3, multiply remainder by 2.
The Year-Item: Add together the number of dozens, the overplus, and the number of 4's in the overplus.
The Month-Item: If it begins or ends with a vowel, subtract the number, denoting its place in the year, from 10. This,
plus its number of days, gives the item for the following month (remember to divide by 7 and keep only the remainder). The item for January is "0"; for
February or March (the third month), "3"; for December (the twelfth month), 12.
The Day-Item: is the day of the month.
The total, thus reached, must be corrected, by deducting "1" (first adding 7, if the total be "0"), if the date be January or February
in a Leap Year: remembering that every year, divisible by 4, is a Leap Year, excepting only the century-years, in New Style, when the
number of centuries is not so divisible (e.g., 1800).
The final result gives the day of th week, "0" meaning Sunday, "1" Monday, and so on.
EXAMPLE: 1783, September 18
17, divided by 4 leaves "1" over; 1 from 3 gives "2"; twice 2 is "4."
83 is 6 dozen and 11, giving 17; plus 2 gives 19, i.e. (dividing by 7) "5." Total 9, i.e. "2."
The item for August is "8 from 10," i.e. "2"' so for September, it is "2 plus 3," i.e. "5." Total 7, i.e. "0," which goes out.
18 gives "." Answer, "Thursday."