Following expression can be used to show last day of the monthin SSRS report.
Format(DateSerial(Year(Parameters!Dataset1_AsPerDate.Value), Month(Parameters!Dataset1_AsPerDate.Value),"1").AddMonths(1).AddDays(-1),"dd/MM/yyyy")
Input: Parameters!Dataset1_AsPerDate.Value = "02/01/2015"
Outout: 31/01/2015
Input: Parameters!Dataset1_AsPerDate.Value = "02/01/2015"
Outout: 31/01/2015