In the original 'Curse of Strahd', the dinner invitation is mentioned several times, but it is mearly a rouse to get the party inside Castle Ravenloft and there is no actual dinner(see Curse of Strahd p.251). For me it needs to be an actual dinner.
In my games, this has become a stand alone D&D session, where I provide the players with dinner and drinks. We spend the evening roleplaying our characters, as DM I take the role of Count Strahd Von Zarovich.
In the long time Strahd has been a dark lord, he has been many adventurers come and go, many have tried to kill him and all have failed. Most don't ever make it to him, before they meet their doom, so he doesn't even raise an eyebrow when a new adventuring party come to Ravenloft. The players need to do something spectacular to get his attention, for me this is killing the Turtle Dragon in lake Zarovich or defeating Winter Splinter. Only then does Strahd send out an invitation.
- from the publisher's blurb