what happens at a prerelease magic party

[Ninja]

what happens at a pre-release magic party

[Arix]

Depends where you go. As far as I know, it's always either draft or sealed deck, including the new set.

In sealed deck, you'll get three boosters of Zendikar and three of Worldwake (using the Worldwake prerelease as an example), you open them up and build a deck out of whatever you get (plus as many basic lands as you need).

Draft is a little more complicated. You get two boosters of Zendikar and one of Worldwake. Open one Zendikar booster, choose a card, and pass the rest to the person on your left. Repeat until all cards are gone. Open the other Zendikar pack, pick a card, and do the same, but pass to the right. Then open the Worldwake booster and do the same, passing to the left again. Then build a deck out of all the cards you picked.

Apparently there's also "open duelling", where you get an intro pack and a booster to modify it with and play with them, but I've never seen it anywhere.