Getting the Date of Some Day of Last Week
This takes advantage of the fact that 1900-01-01 is a Monday. So adding n number of weeks will also be a Monday. 

Here are the datetime values that are being represented by the integers in that expression:

dateadd(week, datediff(week, 7 /*'19000108'*/, getdate()), 0 /*'19000101'*/)


Adding the number of weeks since 1900-01-08 (the week after the first Monday) to 1900-01-01 returns one week less than the current week, i.e. Monday of last week. 

These three are equivalent: 

select dateadd(week, datediff(week, 7 , getdate()), 0)
select dateadd(week, datediff(week, '19000108', getdate()), '19000101')
select dateadd(week, datediff(week, 0, getdate())-1, 0)


rextester demo: http://rextester.com/BQX59903