Can you please explain further to us the formula.

Sure! Happy to try anyway!

The formula A1-WEEKDAY(A1,3) is simply a date calculation.

For example, if you you have today's date (11/13/15) in A1, and in B1 you have =A1-4, the cell returns the date minus 4 days: 11/6/15.

So now that we've established that simply subtracting whole numbers from a date will give you that many days back from the given date, the suggested answer now makes more sense.

The WEEKDAY function takes in 2 input parameters, the date, then "return_type" which according to the tooltip that comes up has several options: 1,2,3 or 11 thru 17. It returns a whole number representing the day of the week, based on the "return_type" you specify. So, for example, WEEKDAY(A1,3) returns the weekday number based on return_type "3", which starts on Monday (0), ends on Sunday (6). Since 11/13/15 is a Friday, and return_type "3" starts counting from 0 on Monday, WEEKDAY(A1,3) = 4.

Continuing on with 11/13/15 as the day we're evaluating, =A1 - WEEKDAY(A1,3) would mean 11/13/15 - 4 = 11/9/15.

So depending on what day of the week you want your week start date to be, you would need to use the WEEKDAY function accordingly.

HTH!