Try this..
may help u.
Int32 day,mon,yr;
//Public declaration
// on any event where u need to generate
BoundField bf = new BoundField();
mon = Calendar1.SelectedDate.Month;
yr = Calendar1.SelectedDate.Year;
day = DateTime.DaysInMonth(yr, mon);
for (int i = 1; i <= day; i++)
{
GridView1.Columns.Add(bf);
}