25 Jan, 1981 = (1980 years + Period from 1.1.1981 to 25.1.1981)
Odd days in 1600 years = 0
Odd days in 300 years = 1
80 years = (60 ordinary years + 20 leap year) = (60 x 1 + 20 x 2)= 2 odd days
Jan = 25 days = (3 weeks + 4 days) = 4 odd days.
Total number of odd days = (0 + 1 + 2 + 4) = 7= 0 odd day.
Given day is Sunday.
