Happy Hour is a very happy event, like the name describes.
If you see a (!!) next to a server name, then there is a happy hour on that server. During a happy hour, your EXP gain is doubled, your stats are all maxed out and you automatically get the Cactus body (only for the hour), and Fred the giant cactus makes a very special appearance. You also have the chance to win a Jump start hat during Happy Hour.
How to get HH for hour Edit
On Kongregate, Spending 50 Kreds in the Vault Of Magics will make a Happy Hour appear on the server you're on.