January 1st of a year is a Monday. What day of week is the 21st of the same month?
January 8th, 15th, and 22nd would also be a Monday as we can add 7 to get the same day of the week. 1 day before Monday is Sunday, the 21st.
Monday = 1 (mod 7)
21 (mod 7) = 0 mod 7 is obviously Sunday given that information.