=Switch(
Fields!Month.Value=1,"January",
Fields!Month.Value=2,"February"
Fields!Month.Value=3,"March"
Fields!Month.Value=4,"April"
Fields!Month.Value=5,"May"
Fields!Month.Value=6,"June"
Fields!Month.Value=7,"July"
Fields!Month.Value=8,"August"
Fields!Month.Value=9,"September"
Fields!Month.Value=10,"October"
Fields!Month.Value=11,"November"
Fields!Month.Value=12,"December"
)