"I have a text box on my form where I enter a date. After I enter a date in text box #1 I need to set the value of text box #2 to be the date of the first Sunday following text box #1.

The exception would be if the date they entered in text box #1 was a Sunday, in which case text box #2 needs to equal text box #1."

Use this in the update event for the 1st box:

Private Sub Text1_BeforeUpdate(Cancel As Integer)
    Dim tmp
    With Me
        tmp = .Text1
        If IsDate(tmp) Then
            Do While DatePart("w", tmp) <> vbSunday
                tmp = DateAdd("d", 1, tmp)
            .Text2 = tmp
        End If
    End With
End Sub