voici la fonction que j'ai programmée

Function semaine(date_s)

année = Year(date_s)
date_premier_an = DateSerial(année, 1, 1)
For j = 0 To 6
date_j = date_premier_an + j
jour = Format(date_j, "dddd", vbMonday)
If jour = "jeudi" Then
date_lun_semaine1 = date_j - 3
If date_s >= date_lun_semaine1 Then
Exit For
Else
date_premier_an = DateSerial(année - 1, 1, 1)
j = -1
End If
End If
Next
semaine = 1 + (date_s - date_lun_semaine1) \ 7

End Function