key attribure of Week Of Month is collection : (Week Of Month,Month,Year)
[Dim Date].[Calendar Hierarchy].[Week Of Month]
WHERE (StrToMember('[Dim Date].[Date].&[2012-03-23T00:00:00]&[03]&[1]&[2012]') :
StrToMember( '[Dim Date].[Date].&[2012-03-31T00:00:00]&[03]&[1]&[2012]'))