Sql Case Expression (Nested)

I am trying to create a worksheet in my Business Intelligence software that is broken down by month. So I created a "calculated column" and added the following sql statement:

Case 
When [vdvVoucher].[PostDate] BETWEEN '2015-01-01T00:00:00' AND '2015-01-31T23:00:00'
then 'January'
Else
Case 
When [vdvVoucher].[PostDate] BETWEEN '2015-02-01T00:00:00' AND '2015-02-28T23:00:00'
then 'February'
Else
Case
When [vdvVoucher].[PostDate] BETWEEN '2015-03-01T00:00:00' AND '2015-03-31T23:00:00'
then 'March'
Else
Case
When [vdvVoucher].[PostDate] BETWEEN '2015-04-01T00:00:00' AND '2015-04-30T23:00:00'
then 'April'
Else
Case
When [vdvVoucher].[PostDate] BETWEEN '2015-05-01T00:00:00' AND '2015-05-31T23:00:00'
then 'May'
Else
Case
When [vdvVoucher].[PostDate] BETWEEN '2015-06-01T00:00:00' AND '2015-06-30T23:00:00'
then 'June'
Else
Case
When [vdvVoucher].[PostDate] BETWEEN '2015-07-01T00:00:00' AND '2015-07-31T23:00:00'
then 'July'
Else
Case 
When [vdvVoucher].[PostDate] BETWEEN '2015-08-01T00:00:00' AND '2015-08-31T23:00:00'
then 'August'
Else 
Case 
When [vdvVoucher].[PostDate] BETWEEN '2015-09-01T00:00:00' AND '2015-09-30T23:00:00'
then 'September'
Else
Case
When [vdvVoucher].[PostDate] BETWEEN '2015-10-01T00:00:00' AND '2015-10-31T23:00:00'
then 'October'
Else
Case
When [vdvVoucher].[PostDate] BETWEEN '2015-11-01T00:00:00' AND '2015-11-30T23:00:00'
then 'November'
Else
Case
When [vdvVoucher].[PostDate] BETWEEN '2015-12-01T00:00:00' AND '2015-12-31T23:00:00'
then 'December'
End
END
End
End
End
End
End
End
End
End
End
End

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I get the following error: Case expressions may only be nested to level 10.

Please advise.