Convert date to first day of the week

[sietze_vergeer-]

Hi Looker users and experts, 

I have a drop out date which is a random day in the week that i want adjust to the first day of the week (Monday). With a table calculation I want to see the inventory level in the drop out week but that data is only linked to the first day of the week.

Basically like the weekday function in excel. 

Anybody any suggestions? Thank you very much