BIP Report template (Alphabetically sorting Issue in pivot table)

 A short description on the problem :

I tried to group the month description above in pivot table , so that it will display the Month Description Horizontally in the pivot . But the output after grouped , will be sorted alphabetically in the order of ascending e.g output "April , Feb , Jan , March ,May ) " or descending "May , March ,Jan ,Feb ,April " which are not suitable to display the month description such order.
So i was wondering how to disable the sorting function or how to customize it , so the display output become " Jan ,Feb , March ,April ,May "