Measure evaluates for all dates only when current week is selected on a slicer

Anonymous , I usually use week rank column

Week Rank = RANKX(all('Date'),'Date'[Week Start date],,ASC,Dense)
OR
Week Rank RANKX(all('Date'),'Date'[Year Week],,ASC,Dense) //YYYYWW format

you can get 6 week

Last 6 weeks = CALCULATE(sum('Table'[Qty]), FILTER(ALL('Date'),'Date'[Week Rank]>=max('Date'[Week Rank])-6 && 'Date'[Week Rank]<=max('Date'[Week Rank])))

Avg Last 6 weeks = CALCULATE(Averagex(Values('Date'[Week Rank]), [orders ]) , FILTER(ALL('Date'),'Date'[Week Rank]>=max('Date'[Week Rank])-6 && 'Date'[Week Rank]<=max('Date'[Week Rank])))

Power BI — Week on Week and WTD
https://medium.com/@amitchandak.1978/power-bi-wtd-questions-time-intelligence-4-5-98c30fab69d3
https://community.powerbi.com/t5/Community-Blog/Week-Is-Not-So-Weak-WTD-Last-WTD-and-This-Week-vs-Last-Week/ba-p/1051123
https://www.youtube.com/watch?v=pnAesWxYgJ8