A fiesta is a holiday with music, dancing, food, and often a church at the centre of it, especially in Spain and Latin America. People pour into the street; it is not a quiet day off. English borrowed the Spanish word as it was, so it still feels Spanish when you say it. People do stretch it, and call any loud party a fiesta, but a birthday in a restaurant is not really one. It grew from the same Latin feast-day word as feast, fest, and fete, then took a Spanish road. You will almost always meet it as a noun. Using it as a verb (“to fiesta”) is playful and rare.