A brief test showed it to work, though it subtracted a week when the date was a Thursday. In other words, a date of 2/5 returns 1/29 rather than 2/5. Not sure if that's what you want. I would have done this, which is what I tested against:
[MyDate]-Choose(Weekday([MyDate],1),3,4,5,6,0,1,2)...