To get the Monday of the current week you can use this formula. To avoid mistakes you can supply the date by using the DATE function such as DATE2020 8 1. TEXT A2 dddd into a blank cell besides your data see screenshot.
We can change the number format to a Fraction to display the number of days.
This should display the day of the week in short form corresponding to the date in A2. To know the exact day name multiply it with 7 with dates. This formula will continue to update on an on-going basis. The formula to return the Start date of the week is as follows.