No, you're not obligated to work holidays without compensation, and that's just an excuse from your boss. Italian law requires that work on holidays be paid with a premium (usually 30-50% more, depending on your sector's collective bargaining agreement) and you're also entitled to compensatory time off. It's true that in retail you often work Sundays, but it can't be done for free and it can't become "normal" just so they avoid paying you. Check your collective bargaining agreement (CCNL for retail) or contact a union: what your boss is telling you is exactly what the law tries to prevent.