I have a Query where I get the WeekDay of a date but by default:
Sunday = 1
Moday = 2
etc.
You can use a formula like:
(weekday + 5) % 7 + 1
If you decide to use this, it would be worth running through some examples to convince yourself that it actually does what you want.
addition: for not to be affected by the DATEFIRST variable (it could be set to any value between 1 and 7) the real formula is :
(weekday + @@DATEFIRST + 5) % 7 + 1