For two years having same calendar must have two common conditions: both having same length in terms of number of days and first day of the week.
The year 1991 has 365 days,,that is 1 odd day , year 1992 has 366 days , i.e. 2 odd days , year 1993 has 365 days i.e. 1 odd day. the years 1994,1995,1996 have 1 odd day each.
the sum of odd days so calculated from year 1990 to 1996 .
(1+2+1+1+1+1)=7 odd days. 5
hence, the year 1997 will have same calendar as that of year 1990.
