First and Last day of a Month C# and VB .NET

DateSerial

Returns a Date value representing a specified year, month, and day, with the time information set to midnight (00:00:00).

Examples
VB.net

' first day of month

FirstDay = DateSerial(Today.Year, Today.Month, 1) 

' month's last day 
LastDay = DateSerial(Today.Year, Today.Month , 0) 

C#

DateTime firstDayOfTheMonth = newDateTime(DateTime.Today.Year,DateTime.Today.Month, 1);

DateTime lastDayOfTheMonth = firstDayOfTheMonth.AddMonths(1).AddDays(-1);