explain my formula and HOW it works (please)

cilck on that cell, then click in teh Fx button to the left of ther formula bar.
A window will pop up showing you the syntax and expected inputs of that formula. click o the WEEDAY part, and then click into the "return type" bar...it will explain to you what it is doing and the expected inputs need to be

You can use this on any function to see what you need to use

If you look in the help for the Weekday formula (click the fx symbol in the formula bar) you'll read that there are three parameter values, 1,2 or 3


With 1 as the parameter the formula will return a value from 1 to 7 where 1 is Sunday and 7 is Saturday
With 2 as the parameter the formula will return a value from 1 to 7 where 1 is Monday and 7 is Sunday
With 3 as the parameter the formula will return a value from 0 to 6 where 0 is Monday and 6 is Sunday

Hence if today is Monday with 3 as the parameter then you are deducting 0 from Today which still leaves the result at Monday

If you want to return the last instance of a specific weekday then you can add a value to TODAY() within the WEEKDAY function, rather than using the second argument of WEEKDAY, in that way you can return the last instance of any day, e.g. this formula will return the last Saturday on or before today

=TODAY()+1-WEEKDAY(TODAY()+1)

Now change +1 at the end to +2 and you get the last Friday on or before today, i.e.

=TODAY()+1-WEEKDAY(TODAY()+2)

Similarly +3 will give the last Thursday, +4 the last Wednesday....and so on