Other forms: celebrants
Someone who's celebrating a happy event is a celebrant. The celebrant, sometimes with the help of friends or family, is the person who gets to blow out the candles on the birthday cake.
You can use celebrant to mean the person being honored at a celebration, or everyone who's participating in it: "The New Year's Eve celebrants cheered as the clock struck midnight." The original meaning of this noun, and the one still most common outside of North America, is "someone who officiates," either at a wedding or a religious service. Celebrant comes from the Latin word celebrare, "assemble to honor."