How to Set DATEFIRST?

[cityboy3]

I have defined a time dimension based on a date field in my fact table (needed a quick and dirty analysis). For my analysis, I need to look at weekly aggregations. However, my company's week starts on Monday. I know that in a stored procedure, I would be able to SET DATEFIRST before using the DatePart function to get a week number. My question is: How do I set this in my dimension definition? In the Dimension Editor, my Member Key Column property looks like this:
DatePart(week,"dbo"."ScheduleOfService"."FromDateTime")
How do I tell it that the week starts on Monday?
thanks.