The Midsummer Fire Festival will run from Thursday, June 21 to Wednesday, July 4 this year. (This year, OverAchiever gives you an effective week's notice on the holiday.) While there aren't actually a lot of achievements associated with the Festival -- the meta awards the Flame Warden title to Alliance players and Flame Keeper title to Horde players but consists of only six achievements -- three of them require you to put in some serious travel time, and one also requires a set of dangerous trips to enemy capitals.
If you're starting from scratch, this is definitely one of the more work-intensive holidays you'll have to clear for What A Long, Strange Trip It's Been. However, the one really good thing about all the work ahead of you is that even if you're starting from scratch, you can afford to get all the rewards from holiday vendors if you're diligent about hitting all the fires, quests, and dailies.
As far as we're aware, there have been no major changes to the holiday apart from the addition of achievements for Northrend and Cataclysm fires, but as always, I'll be around to update and add to the guide in the event that Blizzard springs something new on us. If Blizzard's also added fires to Pandaria on the beta, we'll have a head start on next year's holiday as well.
EDIT: I had a few inaccurate fires that I thought I'd fixed up last year. My apologies, folks -- I've visited them all again this year and made sure the coordinates are accurate as of Cataclysm. I've also rearranged the guides in a rough north-to-south direction to make it easier for anyone who's covering the continents. I hope that helps!