Marion Harland's Complete Etiquette (Unabridged)

The sending and receiving of invitations underlies social obligations. It therefore behooves both senders and recipients to learn the proper form in which these evidences of hospitality should be despatched and received.

In the majority of cases an invitation demands an answer. If one is in doubt, it is well to err on the side of acknowledging an invitation, rather than on that of ignoring it altogether.