I think to have found a solution.

I need to iterate over each member of a set using Existing MDX Function

In this way we can solve the problem of multi select in a Time Dimension...

Existing [Calendario].[DataAAAAMMGG].[MeseVendita].Members,

([Measures].[Num Entrate] - (

ParallelPeriod([Calendario].[DataAAAAMMGG].[AnnoVendita], 1), [Measures].[Num Entrate])))

In this way if i select a Year a 2 Months , I can sum all months of previuos year.

Otherwise PARALLELPERIOD give an error ...

I hope it can be useful for othhers people...

You can use that function also with this example in Adventure Works Cube..

Here is an example...

WITH

MEMBER

[Measures].[Test] AS

SUM

(Existing [Date].[Calendar].[Month].Members,

([Measures].[Order Count] - (

ParallelPeriod([Date].[Calendar].[Calendar Year], 1), [Measures].[Order Count])))

SELECT

{[Measures].[Test], [Measures].[Order Count]}

on columns

FROM

[Adventure Works]

WHERE

({[Date].[Calendar].[Month].&[2004]&[1],

[Date].[Calendar].[Month].&[2004]&[2]})