I think your should be something like....

WITH MEMBER [Measures].[Test] AS

SUM

(

EXISTING [Calendario].[DataAAAAMMGG].[Date].Members,

( [Measures].[Order COUNT] - (ParallelPeriod([Calendario].[DataAAAAMMGG].[AnnoVendita], 1), [Measures].[Order COUNT]) )

)

SELECT{[Measures].[Test], [Measures].[Order COUNT]} ON COLUMNS,

[Calendario].[DataAAAAMMGG].members ON ROWS

FROM [Adventure Works]

Check the [Calendario].[DataAAAAMMGG].[Date] member name though