Using 'Do Until' within "Workbook_Open".

What I am trying to do is on spreadsheet open is;

Checks cell E2 (contains a date)
- If E2 < second date (specified on different sheet within same workbook) perform action
- Move to next row, same column
...and this will loop until E2 >= the second date

I think I am getting close (don't laugh if I am way off) but just can't get it working correctly.
The code I have is....

Private Sub Workbook_Open()
    Dim TestDate As String
    TestDate = ""
    Dim i As Integer
    i = 2
    Dim wks As Worksheet
        With Worksheets("Block")
                Do Until TestDate = "End"
                    If Range(Cells(i, 5)) < Worksheets("Validation").Range("$A$21") Then
                        Range("A" & ActiveCell.Row).Resize(, 13).Interior.ColorIndex = xlColorIndexNone
                        i = i + 1
                    Else
                        TestDate = "End"
                    End If
                Loop

Thank you.