Explanation:

2 years(2 days)+ 1 leap year have(2days) = 4 odd days.
so, 1976- 1979= 4 odd days.
1979 - 1982= 4odd days.
1982 -1985= 4 odd days.
1985 - 1986= 1 odd day.
total odd days = 4+4+4+1 = 13/7= 6 odd days.
=6 odd days. Birthday = Tuesday +6days = Monday.