Try:
DateTime now = DateTime.Now;
DateTime dtFirst = new DateTime(now.Year, now.Month, 1);
DateTime dtTwentieth = new DateTime(now.Year, now.Month, 20);
DateTime test = new DateTime(2011, 12, 17);
if (test >= dtFirst && test <= dtTwentieth)
{
Console.WriteLine("Inside");
}
else
{
Console.WriteLine("Outside");
}