If you're throwing a party and you want it to be fun and fabulous, you're hoping for a festal event. Anything that's festal has a jubilant, celebratory quality.
The adjective festal is a little old-fashioned; these days, you're more likely to use festive instead. The two words are closely related to festival, which was originally defined as "a festal day." All three words share a Latin root, festum, or "feast" — and everyone knows that a feast is at the heart of every truly festal celebration.