One of the commonest feats of lightning calculators down through the ages is to give the day of the week for any date asked for.
Even idiot savants have displayed this ability.
For instance, the twin idiot savants George and Charles, could figure out, virtually instantly, on what day of a week a date fell 2,000 years ago or will fall 2,000 years in the future.
One researcher, memorizing a complex one-page table, tried unsuccessfully to match the speed of the twins for quite a long time.
Then suddenly, one day he discovered he could match their speed! Quite to his surprise, his brain had somehow automated the complex calculations.
It had absorbed the calendar table so efficiently that the calculating process was second nature to him, and he no longer had to consciously go through the various operations.
Perhaps the site of processing migrated from his left brain hemisphere (special- izing in logical, sequential, step-by-step calculating) to his right brain hemisphere (grasping patterns of information in a more holistic and simultaneous fashion).
Autopsies of certain idiot savants that were exceptional in calendar calculating and instant mathematical skills have revealed distinctive neuronal loss in the left hemisphere.
For 19th, 20th and 21st century calendar calculating, memorize the following 2 tables: FOR CALENDAR CALCULATING For any date in the 20th century*- say December 7, 1941 - start with the last two digits of the year Month Table 1 Add (41).
Divide this number by 4 (10 and a remain- January (leap year 0) .
1 der of 1, disregard the remainder, and add the re- February (leap year 3) .
4 sult to the original number (41 plus 10 equals 51) To this sum, add the number corresponding to the March .
4 April .
0 month given in Table 1 (51 plus 6 equals 57).
Add May .
2 the day of the month (57 plus 7 equals 64).
Final- June .
5 ly, divide that total by 7 (9 and a remainder of July .
0 1), but this time pay attention only to the remain- August .
3 der (1).
In Table 2, pick the day corresponding to September .
6 that number.
The answer is Sunday.
*For the 19th century, add 2 to the final total; for October .
1 November .
4 December .
6 the 21st century, subtract 1.
Remainder Tabel 2 Day 0 .
Saturday 1 .
Sunday 2 .
Monday 3 .
Tuesday 4 .
Wednesday 5 .
Thursday 6 .