Automatically Populate Monday before Existing Date in Sheet

Hi all, 

I've seen similar requests on these forums but unfortunately I have not yet been able to make this work. 

I have a project schedule with Start Date and End Date columns. My leadership would like to see two associated columns that show the Monday before the Start Date and End Date to help with planning - so they can see everything starting and coming due in a given week. 

Our initial idea was to simply manually enter this but as dates shift over time and the schedule grows, this will get complicated. 

Can someone help with the formula that will essentially do "If X date, show prior Monday date unless X date is Monday, then just show X date." 


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