Introduction

In order to obtain the day of the week from the date, we can use

 

where k is the day of the month, m is the month ( , ..., , , ), C is century (1990 has C=19), Y is the year (1990 has Y=97 except Y=96 for Jan. and Feb.) W is the day of the week ( , ..., ). (Jan. and Feb. are treated as months of the preceding year). (see [&make_named_href('', "node5.html#zwillinger","[1]")], page 738.) In order to obtain savant's ability, the computation would better be simple and convenient. At least it is required that

• The number of numbers kept by heart is at most ten to twenty.
• The computation does not contain more than two-figure calculations.
• The number of calculation steps is at most few steps.