A festive, happy event, like a big summer backyard party, will be full of gaiety — or merriment and playfulness.
The noun gaiety is the feeling of happiness and delight that might be present at a child's birthday party or a festive square dance. Gaiety can describe a person's generally joyful personality, or the sound of laughter in your house. It comes from the French word gaieté, "joy, merriment, lightheartedness," which evolved from gai, "charming, joyful, happy." The word can be correctly spelled gaiety or gayety, although the first spelling is much more common.