Week-over-week: Week represents last seven days

vminvsky , Assume unique_date is date table, else use data table

Rolling 7
Last Period =
var _max =maxx(Transactions,Transactions[date]) //max date of data collected
var _min = _max -7
var datediff1 = datediff(_min,_max,day)
var _maxX = _max-datediff1
var _minX = _min -datediff1
return
CALCULATE(SUM(Sales[Sales Amount]),filter((date),date[date]<=_max &&date[date]>=_min)) //use all(Date) if needed

Rolling 7 before 7
Last Period =
var _max =maxx(Transactions,Transactions[date]) //max date of data collected
var _min = _max -7
var _maxX = _max-7
var _minX = _min -7
return
CALCULATE(SUM(Sales[Sales Amount]),filter((date),date[date]<=_maxX &&date[date]>=_minX)) //use all(Date) if needed in filter