It should work exactly like this. I would only make sure that the date in is converted properly in CDate call. I've used the following code at it works fine:

Protected Sub DayPilotScheduler1_BeforeCellRender(ByVal sender As Object, ByVal e As DayPilot.Web.Ui.Events.BeforeCellRenderEventArgs) Handles DayPilotScheduler1.BeforeCellRender
  If e.Start = New DateTime(2009, 6, 1) Then
    e.BackgroundColor = "red"
  End If
End Sub

thanks for your reply I've got this working it was the cdate how could I overlook that. Do you know how to get a tooltip on thes "special" dates e.g

If e.Start = New DateTime(2009, 12, 25) Then
e.BackgroundColor = "red"
should generate a tooltip like "Christmas" or (xxxx, 1, 1) "New Year"

The tooltip can be added to the header using BeforeTimeHeaderRender:

1. Detect the second header row using e.IsGroup.
2. Set e.ToolTip.

You would have to modify the source code in order to show tooltips for the actual time cells.

thank you very much that was the answer I needed!

Where would you modify the source code in order to have a tooltip on any cell?