As you have already read by now, July 17 IS
Disneyland's 50th anniversary. I was at the park on the same day in 1995 for the 40th and it was very busy. We were, however able to ride all of the main attractions including Indiana Jones, which was brand new, with only a thirty minute wait.
For the 50th, I would expect it to be a bit busier for a few reasons;
1. The celebration begins on May 5, and is being heavily promoted by Disney.
2. The actual anniversary always draws large crowds of loyal Disney fans from all over.
3. The pin trading craze began a few years ago, and there's bound to be a great deal of people on hand just for that.
4. Since the event is in the middle of the summer season, that will play a big part in the crown.
5. July 17 is also on a Sunday, which doesn't help matters either.
But there are a couple of positive things to consider;
1. Downtown Disney and Disney's California Adventure were not built in 1995. They should help to aleviate the crowd a bit, but not much.
2. Since the anniversary celebration runs through the end of next year, some people might decide to visit at another time.
3. The anticipated fear of the huge crowds will keep some people away.
In any case, I will happily be on hand on July 17th. I've been looking forward to this since 1980, and booked my hotel last summer. Although I am only planning on being a "witness to history" on that day. I will only attend any ceremony that is held and then just take in the park without trying to get on any rides. I'll save that for one of my other days.
Good luck. I highly recommend that you try to be there on the anniversary day so that you can say that you were there.