No matter which NaNo-er you are, let me congratulate you on getting this far. Whether you've written 10k words, or 50k words, that's still a huge chunk of writing to have committed to paper. True, 50k is now well below the threshold considered a 'novel', but hey, you've possibly just written more in one go than you've ever written before.
I'm sure Saturday still feels like a long way off, but what exactly should you do when you hit 50k?
1) Take a break.
You've earned it. Take a couple of days off writing, and do something totally unrelated. Go to the cinema, visit friends, take in a show - anything you want, just don't write. Racking up 50k words in 30 days can take a lot out of you, so you'll need to recharge your batteries if you're going to survive the onslaught that is Christmas. However, at some point, you'll also need to...
2) Finish your novel.
If it's 'finished' at 50k, then you need to decide if you want to keep it as a novella-length piece, or you want to extend it to 80k or more to hit the 'novel' mark. If you want to extend it, consider sub plots, or padding out those scenes that you probably rushed in order to meet the deadline. If you've finished NaNo at 50k but your book itself is still far from complete, then get the rest written. Take your time - the mad panic is over. But one way or the other, get it done.
3) Edit, edit, edit.
Your book is far from finished - it is a first draft, at best. Even if you've typed 'The End' it's still not ready for the public. Put it to one side for a few weeks, maybe even start something new. Put some distance between yourself and your words, so that when you go back to them, you can critically decide what you need to do with what you've produced. Edit it once, and then edit it again. Find a beta reader, or a critique partner - or better yet, find an editor. If you want to put your book out there, it will need to be perfect if it's going to make it in a crowded marketplace.
Now you've finished reading this, get back to your NaNo effort. Only 10k words to go...