=DateSerial(Year(Parameters!ParameterName.Value), Month(Parameters!ParameterName.Value), "1").AddMonths(1).AddDays(-1)
Previous month last date:
=DateSerial(Year(Now()), Month(Now()), "1").AddDays(-1)
Previous month first date:
=DateSerial(Year(Parameters!ParameterName.Value), Month(Parameters!ParameterName.Value), "1").AddMonths(-1)