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OleDbConnection cn2 = new OleDbConnection(dbconnect);
            cn2.Open();
            foreach (DataGridViewRow row in dataGridView1.Rows)
            {
                string grid_kunde = row.Cells[0].Value.ToString();
                string grid_taetigkeiten = row.Cells[1].Value.ToString();
                string grid_stunden = row.Cells[2].Value.ToString();
                string grid_datum = row.Cells[3].Value.ToString();
                string grid_kw = row.Cells[4].Value.ToString();
                string grid_tag = row.Cells[5].Value.ToString();
                string grid_monat = row.Cells[6].Value.ToString();
                string sql_abfrage2 = "INSERT INTO [data_kunden_stunden] (kunde, taetigkeit, stunden, datum, kw, tag, jahr, monat) VALUES ('" + grid_kunde + "', '" + grid_taetigkeiten + "', '" + grid_stunden + "', '" + grid_datum + "', '" + grid_kw + "', '" + grid_tag + "', '" + jahr + "', '" + grid_monat + "')";
                OleDbCommand cmd2 = new OleDbCommand(sql_abfrage2, cn2);
                cmd2.ExecuteNonQuery();
            }
            cn2.Close();					

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uint[] aWeekdays = { 0, 0, 0, 0, 0, 0, 0 };    // Array mit Anzahl der Wochentage (Index 0 = alle Sonntage, 1 = alle Montage etc.)					

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public enum DayType { WorkDay, Weekend, Holiday }

public Dictionary<DateTime, DayType> GetListOfDayTypes(DateTime from, DateTime to)  
{
// TODO
}					

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public Dictionary<DateTime, DayType> GetListOfDayTypes(DateTime from, DateTime to)
{
    Dictionary<DateTime, DayType> result = new Dictionary<DateTime, DayType>();  
    while (from <= to)
    {
        if (from.IsWeekend())
            result.Add(from, DayType.Weekend);
        else if (from.IsFeiertag())
            result.Add(from, DayType.Holiday);
        else
            result.Add(from, DayType.WorkDay);
        from = from.AddDays(1);
    }
    return result;
}					

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foreach (var tuple in GetListOfDayTypes(from, to))
{
    switch (tuple.Value)
    {
        case DayType.WorkDay:
            // Arbeitstag für tuple.Key eintragen
            break;
        case DayType.Weekend:
            // Wochenende für tuple.Key eintragen
            break;
        case DayType.Holiday:
            // Urlaub für tuple.Key eintragen
            break;
        default:
            break;
    }
}