MDX query - need to run different query (where clause) on mondays

Question

The below query works OK and uses the substitution variable CurrentWeek in the WHERE clause, but I have one shortcoming that I need your help for:
The query works fine for all days but mondays. On Mondays I need the query to fetch numbers from last week (as opposed to current week) as the query returns null as the cubes have no data for current week on mondays. Hence I need to change the where clause to use substitution variable LastWeek, or in some other way run a different query on mondays. I have another substitution variable called WeekDay which I can use to test which day of the week it is, but I am not sure of the syntax I need to use to test for day of week and then have the query use different substitution variables as a result of this test.
Please help.
Henning Strand
SELECT NON EMPTY {[Measures].[meas_sg]} ON COLUMNS,
NON EMPTY {[Kunde],[Kunde].[KU01],[Kunde].[KU02],[Kunde].[KU03],[Kunde].[KU04]} ON ROWS
FROM MIFS.MIFS
WHERE ([Marked].[Dagligvare],[Tid].[&CurrentWeek])

Answer 1

Or to go off of Sean's suggestion, programmatically change the value of the substitution variable, and leave the code alone.
Other thoughts -- have a load process to a single cell (sorry, my BSO roots are showing, but the concept is the same, load a single number) and then do a boolean test off its value -- 0 = Monday, 1 = the rest of the week.
Regards,
Cameron Lackpour