You can find Sundays in following way
protected void Page_Load(object sender, EventArgs e)
{
int month = 3;
int year = 2015;
CultureInfo info = new CultureInfo("en-US");
for (int day = 1; day <= info.Calendar.GetDaysInMonth(year, month); day++)
{
if (new DateTime(year, month, day).DayOfWeek == DayOfWeek.Sunday)
{
Response.Write(day + "<br />");
}
}
}