Frostval is an event.
In AdventureQuest a moglin, Chilly, says the holiday of Frostval started there almost a century ago when his great-great-granduncle Hummus thought it would be nice to start a holiday. But "Hummus Day" wasn't very popular, so he gave up making everyone celebrate it. Then Chilly's great-great Aunt Floe decided to cheer Hummus up, she started a holiday everyone loved, she called it Frostval, after their town of Frostvale.
Frostval happens in the winter of every year, and the Moglins of Frostvale build magical items to give to every good person in the land. But Chilly says it ALWAYS seems like BAD people and monsters try to ruin Frostval every year. He also says at least the Hero is here to help them stop them.