Keep an eye on the weather forecast--you'll want to dress comfortably and bring the right gear, lest a rainstorm interrupts the day's headliner. Dark Sky ($4; iOS) is probably the most gorgeous weather app in the App Store, and shows a lot more info than just a simple forecast. It shows you the current temperature, feels-like temperature, and the closest storm to your location. Tap on the temperature for more detail, like wind, humidity, and atmospheric pressure, and keep an eye on Dark Sky's animated radar map to view how things will change over the next few days.
Android and Windows Phone owners, you can install Forecast.io as a bookmarklet on your home screen: This web service is optimized for mobile devices and made by the same developers and pulls the same data as Dark Sky, so the information is just as thorough.
Because festivals are all-day affairs, look into food options before you get there. I spent a fortune on food at Coachella--meals on the festival grounds and groceries at the festival campgrounds' general store were not wallet-friendly. If you're camping, bringing your own food is a good way to cut down on costs. (Just check the campground rules for what types of items can be brought in. Glass containers were a no-no, for example.)
Coleman's Classic Camping Cookbook & Meal Planner(free; iOS) is a handy app to have for preparing campsite cuisine. Select recipes by cooking method (campfire, grill, none), type of meal, or available ingredients for yummy suggestions on things you can make yourself. I just wish it were updated more frequently with more recipes and to better fit the iPhone 5/5s/5c's larger screen.
Not all festivals have camping options--many of them are in metropolitan areas where camping is close to impossible. Hotels can fill up fast if the festival brings in a lot of visitors, so if you find yourself stuck in a city with no place to sleep, Airbnb (free; iOS and Android) can help you out. I used Airbnb to find a place to stay during South by Southwest in Austin last spring.
In case you're unfamiliar with this popular service that has practically pioneered the sharing economy, Airbnb is a network of hosts who have opened part (or all) of their homes to travelers. You can browse the available listings, select a room to rent, and communicate with your host directly through the app. You could also try Couchsurfing (free; iOS and Android), if you don't mind roughing it and possibly sleeping on a stranger's floor, or Hotel Tonight (free; iOS and Android) for last-minute hotel openings.
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