=IF( ISERROR(WEEKDAY(DATEVALUE(CONCATENATE(G14,"-",$BB$9)),2)), 0, WEEKDAY(DATEVALUE(CONCATENATE(G14,"-",$BB$9)),2))

Days    Week
Giorni  Settimana
1       lu (Monday)
2       ma (Tuesday and so on) theoretically correct value
3       me
4       gi
5       ve
6       sa
7       do
0       -- (actually failing value)