Mirth is a formal or literary term meaning fun and enjoyment as shown by laughter. If you and your friends stayed up all night at a sleepover laughing, you might refer to that as a night of mirth.
In 1905, Edith Wharton published a novel "The House of Mirth," whose title derives from Ecclesiastes: "The heart of the wise is in the house of mourning; but the heart of fools is in the house of mirth." But studies show that when human beings laugh, smile or otherwise engage in mirthful activity, our brains release hormones that help us live longer. Mirth may be foolish, but it's good for you.