There are many methods available for determining the day of the week given the day , month and year on that day . However , most methods are suitable for mental calculations . The following formula , which is for the Gregorian Calendar only , may be more convenient for computer programming .
W = ( k + floor function of (2.6 m - 0.2) - 2 *C + Y + floor function of (Y/4) + floor function of (C/4) )mod 7.
- floor function returns the largest integer < = n
- k is day (1 to 31)
- m is month ( 1= March ( it's Gregorian remember) , .........10 = December , 11 = Jan etc)
- C is century ( like for 2016 it is 20 and not 21 ( blimey !))
- Y is year ( 2016 has Y= 16 except 15 for Jan to Feb ( Phew !))
W is weekday ( 0 = Sunday ..............6 = Saturday )
For the pochemuchka , here the century and 400 year corrections are inbuilt into the formula . For some years , the W results into a negative number which is to be corrected by adding 7 to W .