С#. Получение всех дней указанного месяца

Например, так:

var year = 2017;
var month = 2;
var startDay = new DateTime(year, month, 1);
var endDay = startDay.AddMonths(1);
for (var date = startDay; date != endDay; date = date.AddDays(1))
    Console.WriteLine($"Number: {date.Day}, day of week: {date.DayOfWeek}");

int year = 2017, month = 2;
int daysCount = DateTime.DaysInMonth(year, month);
for (int day = 1; day <= daysCount; ++day) {
    string dayName = new DateTime(year, month, day).ToString("dddd");
    Console.WriteLine("Number = {0}, Name = {1}", day, dayName);
}