Microsoft 365 Fusionner colonnes avec formule.

Private Sub Worksheet_Change(ByVal Target As Range)
   If Target.Address = "$A$1" Or Target.Address = "$C$2" Then
      With Me.[C3:AG3]
         .FormulaR1C1 = "=IF(ISOWEEKNUM(OFFSET(R[2]C,0,-1))=ISOWEEKNUM(R[2]C),NA(),ISOWEEKNUM(R[2]C))"
         .Value = .Value
         .SpecialCells(xlCellTypeConstants, xlErrors).ClearContents
         .HorizontalAlignment = xlHAlignCenterAcrossSelection
         End With
      End If
   End Sub

=DATEVAL("1 "&C$2&" "&$A$1)

=LET(Mai;DATE($A$1;5;31);Lun;Mai-JOURSEM(Mai;2);
Lun+7*($C$3-NO.SEMAINE.ISO(Mai))+1)

=SIERREUR(NO.SEMAINE.ISO(DATE($A1;MOIS("1/"&$C2);JOUR(C5)));"")

Private Sub Worksheet_Change(ByVal Target As Range)
If Intersect(Target, [A1,C2]) Is Nothing Then Exit Sub
Dim jour%
Application.ScreenUpdating = False
Application.DisplayAlerts = False
On Error Resume Next
With [C3:AG3]
    .Columns.Hidden = False 'affiche tout
    .UnMerge 'défusionne
    .Formula = "=IFERROR(ISOWEEKNUM(DATE($A1,MONTH(""1/""&$C2),DAY(C5))),"""")"
    For jour = Application.Min(.Cells) To Application.Max(.Cells)
        .Cells(1, Application.Match(jour, .Cells, 0)).Resize(, Application.CountIf(.Cells, jour)).Merge 'fusionne
    Next
    .Borders.Weight = xlThin 'bordures
    For jour = 29 To 31: .Columns(jour).Hidden = .Cells(2, jour) = "": Next 'masque la colonne
End With
End Sub