It is difficult to truly pinpoint when champagne became the drink of reference for upper society parties, but most agree this took place progressively in the 17th century.
The first thing to realize is that wealthy parisian families owned a lot of the land in the Champagne region, so they started to try and make it popular in the French and British court.
The turning point comes in around 1660 when the English invent the resistant glass bottle - now champagne can be aged in glass (better for this type of wine than barrels) which can resist the pressure created when the drink ferments. Quality of the drink is therefore much improved.
Furthermore the wine has limited success in France but is a great success in England. They import it in barrels, and bottle it themselves, adding sugar from their colonies to maximize the "foam" that is produced.
In France, around the end of the century, dom Pérignon (a priest) is credited with developing the first mixes of grapes, and rediscovers old vinification processes. These developments allow for a much more consistent quality year over year, as well as a new variety, giving a champagne for any preference. This sets the stage for the complete adoption of the drink in the 18th century. Mainly put forward by the wealthy elite of Paris (they still owned the land), the women of high society consistently served champagne in their salon littéraire - receptions where the bourgeoises met with philosophers. From then on every court started adopting champagne as their go-to drink for receptions. The Tzar Alexander II even commissioned special flat-bottomed bottles to be used in his court in order to be able to check for bombs in the bottle.
I hope that answers your question somewhat. As I said there is no 1 event we can really point to, but a slow adoption by the aristocracy over a century or two.
Edit: forgot to add sources, sorry :