As,we know that Saturday and Sunday is considered as a weekend,so we can check by using Datetime.DayOfWeek property and DayOfWeek enum value as:-
public static bool IsWeekend(DateTime GivenDate)
// Initialize to true; Means all days are work days.
bool Weekend = false;
// return non work day if saturday or sunday
if (GivenDate.DayOfWeek == DayOfWeek.Saturday || GivenDate.DayOfWeek == DayOfWeek.Sunday)
Weekend = true;