Set date field to null using VBA in Access

How do I set a date to nothing?  For example, a table with a date field initially has nothing, but once a user selects a date, I cannot set it back to blank in VBA (only if user utilizes a form to delete the date.)  I have read that only a variant type can be set to null, but I don't know how to utilize this information to update the actual date field.

Private Sub DATE_COMPLETED_FOR_RELEASE_BeforeUpdate(Cancel As Integer)
Dim dateCompleted As Variant

    dateCompleted = Me.DATE_COMPLETED_FOR_RELEASE.Value
    If Non_Work_Days(dateCompleted) = False Then
       MsgBox "Select a work day, not a Saturday, Sunday or holiday."
         dateCompleted = Null
         Set Me.DATE_COMPLETED_FOR_RELEASE = dateCompleted
        End If
End Sub


Note the code for Non_Work_Days() does work fine and is as below:

Public Function Non_Work_Days(NonBusinessDays As Variant) As Boolean

Dim VarA As Variant
Dim BoolY As Boolean
Dim BoolZ As Boolean

VarA = IsNull(DLookup("Holiday", "t_Holiday_Dates", "Holiday=#" & NonBusinessDays & "#"))
BoolY = Format(NonBusinessDays, "ddd") <> "Sat"
BoolZ = Format(NonBusinessDays, "ddd") <> "Sun"

    If VarA And BoolY And BoolZ = True Then
    Non_Work_Days = True
    Else
    Non_Work_Days = False
    End If

End Function