'For These Two Weeks We Live Like Kings'*
For two weeks each summer, over
10,000 adults (and many children of all ages) come together to play
make-believe. They emerge from their cubicles and their desks
and their workshops to spend two weeks camping out in western
Pennsylvania. 
They pretend that it is the Middle Ages as they Should Have Been. They pretend that they are white knights or rascals or minstrels or ladies or gypsies or wenches; and - by pretending - sometimes become so.
Pennsic is, effectively, a temporary city: there is a large marketplace, there are classes where you
can learn about all aspects of the (current) Middle Ages, 
there are
battles and sporting events, there are camps and coffee shops that are
the equivalent of your neighborhood pub, and there are parties that are
the equivalent of the best nightclub or the worst frat house. It is all
there for your education and enjoyment for two weeks of each summer.
If You Go:
Beware: If you find yourself in a part of this city that is not to your liking, it is your responsibility to take to your feet and find your niche. But also, Be Aware: most folks at Pennsic are good souls, and a request for help will usually be met with a genuine attempt to provide it.
Do Your Research: Visit Pennsic.net. Read it front to back, so to speak. This is the resource for things Pennsic. Visit PennsicWar.org, the official site. Here, you can pre-reg (do, if you can) and find the class schedule, etc.
Garb Up: You can go as simple or fancy as you choose, so long as you make an attempt at period garb. Have fun with this, and remember, the more imaginitive and authentic your garb, the more you contribute to the magic of this event.
Make Connections: The general consensus is that it's better to camp with an established group, if you can. But if you have no SCA connections, you can always put down roots in a singles' camping area.
Drink:
Drink lots of water while you're there. Cooper's Campground water is
generally safe but unpalatable, and bottled water is heavy. Bring a
water filter. In addition to the traditional pitcher-based
filters, you can get ones fitted to individually-sized water bottles.
Eat:
There are some fantastic food vendors, and many camps have meal
plans. But it's easy to forget to eat, and so a good idea to
bring some snacks along: energy bars (esp. mint chocolate chip, for the
really hot days), tinned oysters, honey sticks, and jarred olives are
all good pick-me-ups.
Talk to Strangers:
While you should always use common sense, Pennsic is, as a whole, a
community of really decent, fascinating people. Meet them.
Embrace Contradictions: 
Pennsic is an example of re-creation, not reenactment, so you'll find
Viking women and Elizabethan men
cheering on Ottoman dancers and Celtic
musicians, often all in the same camp. Maybe, the
best way to look at Pennsic is not as an escape into the Middle
Ages, but as a way to bring beauty and grace and ferocity and
seductiveness into our own time. So if you see a samurai carrying a
plastic cup, don't bother to bemoan the plastic: be grateful for
the
samurai.