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January 1st of a year is a Monday.  What day of week is the 21st of the same month?

 Jun 27, 2021
 #1
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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.

 Jun 27, 2021
 #2
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Monday = 1 (mod 7)

21 (mod 7) = 0 mod 7 is obviously Sunday given that information.

 Jun 27, 2021

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