The Midsummer Fire Festival is a time of merriment and festivities celebrating the hottest season of the year! Festival bonfires are lit around all of Azeroth and Outland and players can travel to each to honor (or desecrate) their flames. The festival is a reference to real-world Midsummer celebrations during the summer solstice.
There are a number of things to do during the Midsummer Fire Festival–visit bonfires, toss flaming torches, or battle rogue elemental spirits! Visiting the various bonfires located throughout the world will reward gold and Burning Blossom which can be used to purchase various foods, festive clothing, or even fun toys! Players can also earn Burning Blossom by completing the available festival daily quests. The most skilled of adventurers can choose to delve into The Slave Pens and battle the elemental Ahune.
Completion of all of the holiday achievements specified by The Flame Keeper or The Flame Warden will grant a faction-specific player title: Flame Keeper <Name> or Flame Warden <Name>.
More information on Wowhead.