Upto 12^th May 1975 - 1 odd day

Upto 12^th May 1976 - 2 odd days (As 1976 was a leap year)

Upto 12^th May 1977 - 1 odd day

Upto 12^th May 1978 - 1 odd day

Upto 12^th May 1979 - 1 odd day

Hence from 12^th May 1974 to 12^th May 1979, we have 6 odd days. Now from 13^th May 1979 to 31^st May 1979, we have 19 days i.e. 5 odd days. In June we have 2 odd days and in July and August there are 3 odd days. In September, we have 21 days till 21st September. So there is no odd day in September. Hence the total odd days are 6 + 5 + 2 + 3 + 3 = 19 or 5 odd days. Hence from 12^th May 1974 to 21^st September, 1979, we have 5 odd days. Since 12^th May 1974 was Saturday, hence 21^st September, 1979 will be Thursday.