Public Class Form1
    Private mDT As DataTable
    Private WithEvents mBS As BindingSource
    Private WithEvents DGV As DataGridView
    Private WithEvents btnAddNew As Button
    Private Rnd As New Random
 
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        CreateControls()
        CreateData()
 
        With DGV
            .DataSource = mBS
            .Columns(0).DefaultCellStyle.Alignment = _
            DataGridViewContentAlignment.MiddleRight
            .Columns(2).DefaultCellStyle.Format = "dd.MM.yyyy"
            .Columns(2).DefaultCellStyle.NullValue = ""
            .Columns(3).DefaultCellStyle.Format = "HH:mm"
            .Columns(3).DefaultCellStyle.NullValue = "00:00"
            .Columns(4).DefaultCellStyle.Format = "HH:mm"
            .Columns(4).DefaultCellStyle.NullValue = "00:00"
            .AutoResizeColumns()
            .AutoResizeRows()
        End With
        mBS.Position = mBS.Count - 1
    End Sub
 
    Private Sub btnAddNew_Click _
(ByVal sender As Object, _
ByVal e As System.EventArgs _
) Handles btnAddNew.Click
 
        Dim D As Date = _
        Date.Today.AddDays(Rnd.Next(0, 1))
 
        Dim DR As DataRow = mDT.NewRow
        Dim NewID As Integer = GetNewID()
        DR.Item(0) = NewID
        DR.Item(1) = Format(D, "ddd")
        DR.Item(2) = D
        DR.Item(3) = Format(Now, "HH:mm")
        DR.Item(4) = Format(Now, "HH:mm")
        mDT.Rows.Add(DR)
 
        Dim Pos As Integer = _
        mBS.Find("ID", NewID)
 
        mBS.Position = Pos
    End Sub
 
    Private Sub CreateData()
        Dim i As Integer
        Dim DR As DataRow
 
        mDT = New DataTable
        With mDT
            .Columns.Add("ID", GetType(Integer))
            .Columns(0).AllowDBNull = False
            .Columns(0).Unique = True
            .Columns.Add("Tag", GetType(String))
            .Columns.Add("Datum", GetType(Date))
            .Columns.Add("Start", GetType(Date))
            .Columns.Add("Ende", GetType(Date))
            Dim D As Date
            For i = 1 To 15
                D = Date.Today.AddDays _
                (Rnd.Next(0, 5))
 
                DR = .NewRow
                DR.Item(0) = i
                DR.Item(1) = Format(D, "ddd")
                DR.Item(2) = D
                DR.Item(3) = Format(Now, "HH:mm")
                DR.Item(4) = Format(Now, "HH:mm")
                .Rows.Add(DR)
            Next
            .AcceptChanges()
        End With
 
        mBS = New BindingSource
        mBS.DataSource = mDT
        mBS.Sort = "ID"
    End Sub
 
    Private Function GetNewID() As Integer
        Dim DV As DataView = _
        DirectCast(mBS.List, DataView)
 
        Dim DRV As DataRowView
        Dim ID As Integer
        For Each DRV In DV
            If DRV.Item(0) IsNot DBNull.Value Then
 
                Dim i As Integer = _
                CType(DRV.Item(0), Integer)
 
                If i > ID Then
                    ID = i
                End If
            End If
        Next
        Return ID + 1
    End Function
 
    Private Sub CreateControls()
        Me.Size = New Size(650, 670)
        DGV = New DataGridView
        With DGV
            .SetBounds _
            (10, 10, _
            Me.ClientSize.Width - 20, _
            Me.ClientSize.Height - 70)
 
            .Anchor = AnchorStyles.Left Or _
            AnchorStyles.Top Or _
            AnchorStyles.Right Or _
            AnchorStyles.Bottom
 
            .DefaultCellStyle.Font = _
            New Font("Arial", 12)
 
            .ColumnHeadersDefaultCellStyle.Font = _
            New Font("Arial", 8, FontStyle.Bold)
        End With
        Me.Controls.Add(DGV)
 
        btnAddNew = New Button
        With btnAddNew
            .SetBounds _
            (Me.ClientSize.Width - 110, _
            DGV.Bottom + 10, _
            100, 40)
 
            .Anchor = AnchorStyles.Right Or _
            AnchorStyles.Bottom
 
            .Text = "Add New"
        End With
        Me.Controls.Add(btnAddNew)
    End Sub
 
    Private Sub mBS_PositionChanged _
    (ByVal sender As Object, _
    ByVal e As System.EventArgs _
    ) Handles mBS.PositionChanged
 
        If mBS.Position > -1 Then
            Dim DR As DataRow
            Dim DRV As DataRowView
 
            DRV = DirectCast(mBS.Current, DataRowView)
            DR = DRV.Row
 
            Me.Text = DR.Item(0).ToString & "; " & _
            DR.Item(1).ToString & "; " & _
            DR.Item(2).ToString
        End If
    End Sub
End Class

mDT = New DataTable
With mDT
    .Columns.Add("ID", GetType(Integer))
    .Columns(0).AutoIncrement = True
    .Columns(0).AllowDBNull = False
    .Columns(0).Unique = True
 
    .Columns.Add("Tag", GetType(String))
    .Columns.Add("Datum", GetType(Date))
    .Columns.Add("Start", GetType(Date))
    .Columns.Add("Ende", GetType(Date))
    .Columns.Add("Pause", GetType(Integer))
    .Columns.Add("Soll AZ", GetType(Integer))
    .Columns.Add("Bemerkung", GetType(String))
    .AcceptChanges()
End With

Private Sub btnAddNew_Click(ByVal sender As Object, ByVal e As _
  System.EventArgs) Handles btnAddNew.Click
 
    Dim DateFrom As Date = New Date(2019, 9, 16)
    Dim DateTo As Date = New Date(2019, 9, 22)
    Dim n As Integer = CInt((DateTo.Date - DateFrom.Date).TotalDays)
 
    For i As Integer = 0 To n
 
        Dim D As Date = DateFrom.Date.AddDays(i)
 
        Dim DR As DataRow = mDT.NewRow
        'DR.Item(0) = Nothing
        DR.Item(1) = D.ToString("ddd")
        DR.Item(2) = D
 
        If D.DayOfWeek = DayOfWeek.Saturday Or D.DayOfWeek = DayOfWeek.Sunday _
          Then
            DR.Item(3) = D
            DR.Item(4) = D
            DR.Item(5) = New TimeSpan(0, 0, 0).TotalMinutes
            DR.Item(6) = New TimeSpan(0, 0, 0).TotalMinutes
            DR.Item(7) = "WE"
        Else
            DR.Item(3) = D.Add(New TimeSpan(7, 30, 0))
            DR.Item(4) = D.Add(New TimeSpan(16, 0, 0))
            DR.Item(5) = New TimeSpan(0, 45, 0).TotalMinutes
            DR.Item(6) = New TimeSpan(6, 48, 0).TotalMinutes
            DR.Item(7) = "Anwesend "
        End If
 
        mDT.Rows.Add(DR)
    Next
 
End Sub

...
For i As Integer = 0 To n
    Dim D As Date = DateFrom.Date.AddDays(i)
    'Abfrage ob Datum schon vorhanden ist
    If mDT.Select("Datum = '" & D & "'").Count = 0 Then 
        Dim DR As DataRow = mDT.NewRow
        ...
        mDT.Rows.Add(DR)
    End If
Next
...