For i As Integer = 0 To DGV.Rows.Count - 1
 
            kalender_1.SelectionStart = CType(DGV.Rows(i).Cells(11))
            kalender_1.SelectionEnd = CType(DGV.Rows(i).Cells(12))
 
        Next

  'Beispieldaten in Tabellenspalte 
  Dim dt As New DataTable
  dt.Columns.Add("Date", GetType(DateTime))
  dt.Rows.Add(New Date(2024, 1, 1))
  dt.Rows.Add(New Date(2024, 1, 5))
  dt.Rows.Add({Nothing})
  dt.Rows.Add(New Date(2024, 1, 15))
 
  'Datumswerte fett darstellen (mc ist ein MonthCalendar)
  For r As Integer = 0 To dt.Rows.Count - 1
      If Not IsDBNull(dt.Rows(r)("date")) Then
          mc.AddBoldedDate(DirectCast(dt.Rows(r)("Date"), Date))
      Else
          'was auch immer
 
      End If
  Next r
 
  mc.UpdateBoldedDates()

 Private Sub dateneinlesen(Datum_von As SelectionRange, Datum_bis As _
   SelectionRange)
 
        Dim dt As New DataTable
        dt.Columns.Add("Datum_von", GetType(DateTime))
        dt.Columns.Add("Datum_bis", GetType(DateTime))
 
        For r As Integer = 0 To dt.Rows.Count - 1
            If Not IsDBNull(dt.Rows(r)("Datum_von")) Then
 
                kalender_1.SelectionRange = Datum_von
                kalender_1.SelectionRange = Datum_bis
 
            End If
     end sub

 Dim DT1 As TermineDataSet.DT1Row
 
        For i As Integer = 0 To DT1.Row.Count - 1
 
            If Not IsDBNull(DT1.Rows(i)("Datum_von")) Then
                kalender_1.SelectionStart = CDate("Datum_von")
                kalender_1.SelectionEnd = CDate("Datum_bis")
            End If
        Next i

Public Class frmMonthCalendarDemo
 
    Dim mc As New MonthCalendar With {.Parent = Me, .Size = New Size(400, 400)}
 
 
    Private Sub frmMonthCalendarDemo_Load(sender As Object, e As EventArgs) _
      Handles MyBase.Load
 
        'Eine Beispieltabelle 
        Dim dt As New DataTable
        dt.Columns.Add("Datum_Von", GetType(DateTime))
        dt.Columns.Add("Datum_bis", GetType(DateTime))
        'Zwei Zeitspannen
        dt.Rows.Add({New Date(2024, 1, 1), New Date(2024, 1, 7)})
        dt.Rows.Add({New Date(2024, 1, 13), New Date(2024, 1, 19)})
        dt.Rows.Add({Nothing, New Date(2024, 1, 31)})
 
        Dim d As DateTime
 
        For r As Integer = 0 To dt.Rows.Count - 1
            If Not IsDBNull(dt.Rows(r)("Datum_von")) _
               AndAlso Not IsDBNull(dt.Rows(r)("Datum_Bis")) Then
 
                d = CType(dt.Rows(r)("Datum_von"), DateTime).Date
                'Tagesschleife über die Zeitspanne
                Do
                    mc.AddBoldedDate(d)
                    d = d.AddDays(1)
                Loop While d <= CType(dt.Rows(r)("Datum_bis"), DateTime).Date
            End If
 
        Next r
 
        mc.UpdateBoldedDates()
    End Sub
End Class

 Dim dt As New DataTable

Dim DT1 As DataTable

 Dim dt As DataTable = DT1