OK, I think I know what you need.
WITH MEMBER [Dim2].[Date].[MM]
AS
' Aggregate( { [Dim2].[Date].[2007-12-31], [Dim2].[Date].[NULL] } ) '
SELECT
{ [Measures].[A], [Measures]. } ON COLUMNS,
{ [Dim1].[Hierarchy].Children } ON ROWS
FROM
[CUBE]
WHERE
( [Dim2].[Date].[MM] )
Something like that....