Wiktionary
n. A private viewing of an art exhibition before it opens to the public.
Wikipedia
A vernissage (from French, originally meaning “ varnishing”) is a term used for a preview of an art exhibition, which may be private, before the formal opening. If the vernissage is not open to the public, but only for invited guests, it is often called a private view.
At official exhibitions in the nineteenth century, such as the Royal Academy summer exhibition, artists would give a finishing touch to their works by varnishing them. The custom of patrons and the élite of visiting the academies during the varnishing day prior to the formal opening of the exhibition gave rise to the tradition of celebrating the completion of an art work or a series of art works with friends and sponsors. In the twentieth century it became an opportunity to market the works on view to buyers and critics.
There is a comparable ceremonial ending of art exhibitions, called a finissage. Larger art exhibitions also may have an event at half time of the exhibition called a midissage. These latter terms are rare in English; they are more commonly used in German but not in French.
Usage examples of "vernissage".
The day before the vernissage Tony Cox, the gallery manager Genevieve Morgan, Miles and his wife Sue, and a few others were sitting around in the basement as Yoko put the finishing touches to the show.
He handed the girl a jar of ointment, happy to be present at a vernissage no larger than the skin area of a typist.
I've got to run, Bunker's persuaded me to press charges against that pitiful creature who threw the catsup on my sables when we came out of the clinic thank God it wasn't the chinchilla Mummy would kill me so I'll miss the vernissage for what's his name I can't pronounce it, are you going?