WITH
SET [S_PrevDate] AS
    StrToMember(@Month, CONSTRAINED).PrevMember

MEMBER x AS
    SetToStr([S_PrevDate])

SELECT x ON 0 FROM [cube]

WHERE StrToMember(@Month, CONSTRAINED)

WITH
SET [S_PrevDate] AS
    [Date].[Y-Q-M-D].[Month].&[201708]    -- August

MEMBER x AS
    SetToStr([S_PrevDate])

SELECT x ON 0 FROM [cube]

WHERE [Date].[Y-Q-M-D].[Month].&[201709]    -- September