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The days of the week, starting with Sunday, have indices 0,1,..,.,6. The months of the year, starting with January, are numbered 1,2,...,12. The following pattern holds for common (non-leap) years.
I(n) = I + a(n) (mod 7), n=1,..,.,12, with I the index of January 01 in a common (non-leap) year, and I(n) the index of the day of the week of the first day of the n-th month in this year.
In the year 2011 Jan 01 has index 6 (Saturday). Therefore, Feb 01 has index 6+3 = 2 (mod 7) (Tuesday), Mar 01 has also has index 2, Apr 01 has index 6+6 = 5 (mod 7) (Friday), etc.
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If Jan 01 falls on a day of the week with index I, then Feb 01 is on the day with index I+3 (mod 7), Mar 01 is also on the day with index I+3 (mod 7), Apr 1 01 is on the day with index I+6 (mod 7), etc.
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In the year 2011 Jan 01 has index 6 (Saturday). Therefore, Feb 01 has index 6+3 = 2 (mod 7) (Tuesday), Mar 01 has also index 2, Apr 01 has index 6+6 = 5 (mod 7) (Friday), etc.
Mar 01 has also index 2, Apr 01 has index 6+6 = 5 (mod 7) (Friday), etc.
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