What was the day of the week on 15 May 1989?

\(\frac{Date + Month\;code + Century\;code + Century\;upper\;year + Quotient\;of\;\frac{upper\;year}{4}}{7}\) 

\(\frac{15 + 1 + 0 + 89 + 22}{7}\) = \(\frac{127}{7}\) = Remainder 1

The day for Code 1 is Monday.

Note:     

Code for      Mon – 1      Tues – 2      Wed – 3      Thurs – 4

                   Fri – 5         Sat ­­– 6        Sun – 0

Code for      Jan – 0        Apr – 6       July – 6       Oct – 0

                   Feb – 3       May – 1      Aug – 2       Nov – 3

                   Mar – 3       June – 4      Sep – 5       Dec – 5

Code for      1900 is 0

2000 is 6

2100 is 4

2200 is 2

2300 is 0

2400 is 6

2500 is 4   and  Likewise