RSVP. Repondez s’il vous plait. Please reply.
‘High society’ adopted French etiquette in the 18th century, leading to the common RSVP on invitations. If an invitation simply states RSVP, you should reply within a day or two of receiving an invitation. This is a primary example of WHY invitations are not to be sent months in advance of an event. In the case of larger events, such as weddings, a ‘save the date’ may be sent.
Commonly now, ‘regrets only’ is on the invitation for less formal events. Personally, I’m not a fan, nor was Emily Post (though the same cannot be said for her granddaughter-in-law, Peggy Post). If this is on the invitation instead of the traditional RSVP, it’s exactly as it seems: you only reply if you are unable to attend.
Do you have any specific RSVP questions? Feel free to ask!