Algorithm used to calculate the day of a week

W = (D + 2 * m + 3 * (m + 1)/5 + Y/4-y/100 + Y/400) mod 7
In the formula, D indicates the date in the date, M indicates the number of months, and y indicates the number of years.

Note: In this formula, and are regarded as the 13th and 14th months of the previous year. For example, if-1-10 is used, the formula is converted to 2003-13-10 for calculation.

Below is C #'sCodeImplementation

Code
///   <Summary>
/// Calculate the week of the input date
///   </Summary>
///   <Param name = "year"> Year, yyyy format: 2009 </Param>
///   <Param name = "month"> Month: an integer between 1 and 12 </Param>
///   <Param name = "day"> Date: an integer between 1 and 31 </Param>
///   <Returns> The number of weeks. One to seven represents Monday and seven represents Sunday. </Returns>
Int Week ( Int Year, Int Month, Int Day ){

If (Month =   1   | Month =   2 ){
Month + =   12 ;
Year -=   1 ;
}

Int Week = (Day +   2   * Month +   3   * (Month +   1 ) /   5   + Year + Year /   4   - Year /   100   + Year /   400 ) %   7 ;
Return (Week +   1 );
}