Clothing is an important part of the medieval experience. You will be expected to remain in medieval garb from the time you finish unpacking to the time you repack to go home. Some mundane clothing is tolerated, but is isn't encouraged. Plan to have an outfit a day for the length of your stay, or plan to do laundry. The weather is hot and while body odor is, in fact, period, that doesn't make it acceptable to your campmates.
Most of the time, Pennsic is HOT. It is, after all, mid-August. Save your pretty court garb for winter events, or, at least, save it for evening courts when it's cooler. Cool, lightweight, camp gear is the uniform of the day and can be anything you or your persona requires as long as it fits into the 500 - 1600 C.E. time period. Men generally wear lightweight tunics with pants for daywear (Many do without the pants. Yes it's period!) Some strip down to bunny fur G-strings and loin cloths, but if you plan to do this, remember that sunburn can be a killer! The same goes for women. The more you expose, the more can get burned on the mostly-treeless campground. Most women choose cool, cotton long or short tunics. Many choose to wear harem-type pants with a sleeveless jacket or short tunic. All of these are fine for daywear. Don't forget to bring a head covering. Veils or straw hats are fine for women and men often wear straw hats or Arabic kafias. No one at Chirurgeon's Point want to see you come in with a third-degree sunburn! Footwear is hardest to make period. Sandals are almost always fine. No one will look askance at a pair of leather sandals for either men or women. Tevas are also very practical since they are easily washed off. They don't look as period, but no one will notice when your shoes are covered with a layer of dust or mud. Moccasins of any sort will serve quite well, and the high, soft leather boots are common garb for both men and women. White, hightop sneakers are right out! If you must wear sneakers for health reasons, try to find dark colored ones. Dark-colored leather boots (military or hiking) are tolerated, albeit non-period, especially if they are mostly covered by your pants. For fighting, modern boots are quite acceptable.
We mentioned that most of the time Pennsic is hot and dusty. That's except when it's cold and wet. Be prepared for changes in the weather. The temperature at night can drop into the 40's and a cloak can be a very welcome item of clothing. Heavier shoes or boots are also a good idea for bad weather, or for fighting. Period or not, no one wants to be caught on a cold, rainy night with only wet sandals for warmth. Plan to keep a change of durable mundane garb, including shoes, in a plastic bag or bin. If storms happen, your tent could be washed out leaving you with no dry clothing. Better to be safe than sorry.
A word to the wise for the ladies: Pantaloons, bloomers or shorts to be worn under your dresses are a good idea, especially if you tend to be a little fuller of figure. If you don't understand why, please ask before you have a very uncomfortable experience. (I've been told lately that shorts are also a good idea for men. Take heed, gents.)
In short, dress at Pennsic as if you were attending a medieval picnic. Be safe, be comfortable and make an effort to be period. There will be ample opportunities to wear more elaborate garb in the evenings for court, so don't hesitate to bring it. Just don't expect to rely only on your event garb.