If the written invitation bears the initials RSVP (Repondez s'il vous plait), it must be answered.
Other written invitations will carry the host's telephone number so you can call with your reply, although under strict etiquette rules, a written invitation requires a written reply.
Etiquette rules followed in most Western cultures require that if you receive a formal, written invitation, you should reply promptly, perhaps that same day.
