WoWDB

The annual world event is considered the in-game representation of the United States of America's celebration of Independence Day (a.k.a. the Fourth of July).

It used to be called Engineers' Explosive Extravaganza but was incorporated into the tail-end of the Midsummer Fire Festival and renamed to Fireworks Spectacular.  The fireworks begin after sunset (6 pm server time, usually) and can be seen in any capital city and Booty Bay on the hour every hour until the end of the event.  Booty Bay probably offers the best view of the fireworks since it is more open and also dark.  There are also tables with Festive Mugs that can be filled by using nearby Festive Kegs.  Drinking a  Filled Festive Mug will cast  Lightheaded on your character... which does indeed act as a  Slow Fall.  (Filled Festive Mug may only be used outside of combat.  It may be used in battlegrounds, don't know about arenas.)

/dance with a reveler to gain  Celebrate Good Times!

The lore of the event according to Wowpedia:

Deep within the cavernous halls of the Undermine, goblins labor and toil over their strange and fanciful engineering creations. When they are not constructing powerful devices of destruction, they find making fireworks a diverting pastime. They work tirelessly over strange chemical concoctions used to produce precisely the right color and, of course, spectacular explosions. It is an art form that comes naturally to those who find great pleasure in making things explode in a variety of ways. Competition is fierce among the engineers as they strive for perfection within their pyrotechnics. Since holding an exhibition indoors — particularly within range of a goblin engineer's workshop — is completely out of the question, it was decided by the Tinkers' Union that a day would be chosen wherein the pyrotechnicians would go out into the world and share their creations with the world (for a modest fee, of course). For one glorious day, the sky would be alight with the fantastic creations of engineers throughout Azeroth.