Something like this would work:

aggregate

(

{[Date].[Fiscal Date Hierarchy].currentmember : [Date].[Fiscal Date Hierarchy].currentmember.lead(11)}

,[Measures].[Sales]

)

Note that this would include the current month, and the next 11 months...making it 12 in total.

If you need to exclude the current month, it would be as simple as this:

aggregate

(

{[Date].[Fiscal Date Hierarchy].currentmember.lead(1) : [Date].[Fiscal Date Hierarchy].currentmember.lead(12)}

,[Measures].[Sales]

)

Hope this helps.