Hi:
SheetPlan should be worksheets("Plan")

Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As
Boolean)
If SheetPlan.Range("A1") = DateSerial(2007, 3, 5) Then
'  XXXXXXXX
   Worksheets("Plan").Range("B43") =
Worksheets("Summary").Range ("E14")
End If
End Sub

for multiple dates try an if statement:

Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, _
 Cancel As Boolean)
dim dc as date  ' hold the date from the worksheet
dim sCellAddress as string ' hold the address of the cell

dc=worksheets("Plan").Range("A1")

If dc=  DateSerial(2007, 3, 6) Then
   sCellAddress="B43"
elseif  dc=  DateSerial(2007, 3, 7) Then
   sCellAddress="B44"
elseif dc=  DateSerial(2007, 3, 8) Then
   sCellAddress="B45"
elseif dc=  DateSerial(2007, 4, 1) Then
   sCellAddress="B46"
End If
Worksheets("Plan").Range(sCellAddress) =
Worksheets("Summary").Range ("F14")

End Sub

But I would rather use a lookup table in the workbook say on sheet called
'lookup' with vba where in your lookup table you have

date                           destination_cell         destination_worksheet
=date(2007,3,6)         B43                          Plan
=date(2007,3,7)         B44                          Plan
=date(2007,3,8)         B45                          Plan
=date(2007,4,1)         B46                          Plan

you can then do:

Const csz_wsLookupName As String = "lookup"
Const csz_rLookup As String = "A:C"
Const ci_ColCell As Long = 2
Const ci_ColSheet As Long = 3
'Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, _
' Cancel As Boolean)

Private Sub Workbook_BeforeSave( _
ByVal SaveAsUI As Boolean, Cancel As Boolean)
Dim dc As Date  ' hold the date from the worksheet
Dim sz_CellAddress As String ' hold the address of the cell
Dim sz_Sheet As String ' hold sheet name
Dim a ' answer
dc = Worksheets("Plan").Range("A1")
a = Application.VLookup(CLng(dc), _
Worksheets(csz_wsLookupName).Range(csz_rLookup), _
ci_ColCell, False)
If Not (IsError(a)) Then
   sz_CellAddress = a
   a = Application.WorksheetFunction.VLookup(CLng(dc), _
   Worksheets(csz_wsLookupName).Range(csz_rLookup), _
       ci_ColSheet, False)
   sz_Sheet = a
   Worksheets(sz_Sheet).Range(sz_CellAddress) = _
   Worksheets("Summary").Range("F14")
End If

End Sub