A user on another forum suggested this:

DATEADD(dd,-DATEDIFF(dd,2, YourDateColumn)%7,CAST(YourDateColumn AS DATE))

DATEDIFF(dd,2,YourDateColumn) calculates the number of days between the value in YourDateColumn and the date represented by the number 2 - which is Jan 3, 1900 - which happened to be a Wednesday.

So DATEDIFF(dd,2,YourDateColumn)%7 will be zero if YourDateColumn is a Wednesday, 1 if it is Thursday, 2 if it is Friday and so on.

When you subtract 0 days from Wed, or 1 day from Thursday, or 2 days from Friday and so on, you get to Wednesday. Hence the "-DATEDIFF(dd,2,YourDateColumn)%7".