Calendar

This trick must be used when we need to calculate previous date from the given date.

1 January 1982 was a Friday, What day of the week will be on 1 January 1982

1982 is a ordinary year = 1 odd day
1983 is a ordinary year = 1 odd day
1984 is a leap year = 2 odd days
1985 is a ordinary year = 1 odd day

total odd days = 5 odd days

1 January 1982 was a Friday + 5 odd day

1 January 1986 = Wednesday