First off I would like to thank Bob Askew for some of this code (I cannibalized it from the Get Month Number From Month Name code he provided).

Second, you would use this either passing the full day name or an abbreviation:

'code starts here:

Function GetFirstOfDay(pstrDay As String) As Date
    Dim strStartDay As String
    Dim strDays As String
    Dim intPos As Integer
    Dim intDay As Integer
    Dim strHold As String

    strDays = "SunMonTueWedThuFriSat"
    strHold = Left(pstrDay, 3)
    intDay = InStr(strDays, strHold)

    strStartDay = Format(DateSerial(Year(Date), Month(Date), 1), "dddd")

    If strStartDay = pstrDay Then
        GetFirstOfDay = DateSerial(Year(Date), Month(Date), 1)
    Else
        GetFirstOfDay = DateSerial(Year(Date), Month(Date), 1 + ((intDay \ 3 + 1) - 2))
    End If
End Function