Explanation : There are two conditions for two years having the
same calendar: both having same length in terms of
number of days and first day starting with same day of the week.
The year 1991 has 365 days, that is, 1 odd day, and
year 1992 has 366 days, that is, 2 odd days, while year
1993 has 365 days, that is, 1 odd day. The years 1994,1995, and 1996 have 1 odd day each.
The sum of odd days so calculated from year 1991 to
1996.
(1 + 2 + 1 + 1 + 1 + 1) = 7 odd days.
Hence, the year 1997 will have the same calendar as that of the year 1991.