What is an adjective for merriment?

Formal:

* Jovial

* Joyful

* Merry

* Cheerful

* Exuberant

* Blithe

* Jubilant

Informal:

* Fun

* Happy

* Lively

* Festive

* Giddy

* Spirited

* High-spirited

The best adjective to use will depend on the specific context.