int GecerliHafta = System.Globalization.CultureInfo.CurrentUICulture.Calendar.GetWeekOfYear(DateTime.Now, System.Globalization.CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
int Ay = DateTime.Now.Month;
int Yil = DateTime.Now.Year;
int Count = DateTime.DaysInMonth(Yil, Ay);
DataTable dt = new DataTable();
DataColumn dc = new DataColumn("Tarih");
dt.Columns.Add(dc);
DataRow dr;
for (int i = 1; i < Count + 1; i++)
{
    int Hafta = System.Globalization.CultureInfo.CurrentUICulture.Calendar.GetWeekOfYear(new DateTime(Yil, Ay, i), System.Globalization.CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
    if (GecerliHafta == Hafta)
    {
        dr = dt.NewRow();
        dr["Tarih"] = new DateTime(Yil, Ay, i);
        dt.Rows.Add(dr);
    }
}
//DataTable(dt) içerisinde belirtilen günün(DateTime.Now) bulunduğu haftanın günleri vardır.