You can get the start of the week to start on Monday by creating a calculated field similar to the following:
DATEADD('day', 1, DATETRUNC('week',DATEADD('day', -1, [Order Date])))
The following thread goes into a little more detail:
Hope this helps!
since recently I use Tableau and had exact same question. Thank you for your suggestion.it worked
is there also a possibility to set week beginning to Monday 8 am?