A vernissage (varnishing, from French), also known as a preview or private view, is the ceremonial start of an art exhibition. It is usually a social event that people attend to show up, strolling around with glasses of free sparkling wine and canapés, talking to artists about the works in the exhibition.
At official exhibitions, such as the Royal Academy summer exhibition, artists, in the past, would give a finishing touch to their works by varnishing them (J M W Turner was known for actually starting and completing works on varnishing day while his fellow academicians were simply varnishing). 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. Nowadays for commercial shows it is an opportunity to market the works on sale to influential buyers and critics.
There also is a comparable ceremonial ending of art exhibitions. Bigger art exhibitions also have such an event at half time of the exhibition.