Q:
A: There's nothing special. Simply say "Yes, I look forward to attending", or "I regret that I will be unable to attend, but thank you for the invitation". If you want to add something extra, you also say "Mazel Tov", which is a Hebrew expression for offering congratulations on special occasions. A gift is up to you, depending upon your relationship to the family that offered you the invitation. If you want to send a gift, a check or a gift card to the young man or young lady are appropriate.