I remember reading RUMORS (absolutely not fact at this point) of a 60-hour party for
Disneyland's 60th. I don't even know how that would work and even IF Disney were considering it, it may not happen at all.
I figure rumors are just guesses of what might or might not happen. But if we're already speculating about unknown crowd levels, we might as well speculate about unknown events that might or might not affect those unknown crowd levels.

For instance, if all things are equal but you're
completely horrified at the idea of attending during a 60-hour party, you might consider moving your dates away from the official date of the 60th, just in case the party ends up happening.
If it were my trip, I'm not sure how I'd feel about a 2.5-day party which may or may not occur. In fact, if I were booking at this moment, all I know for sure is that I would personally not plan to
start my trip on the Saturday, Sunday or Monday after the 17th...it would seem weird to start a trip the day/s immediately following the 60th, and if a party did occur, any party crowds and/or closures for party cleanup would be in full swing on those days, even though you didn't get to enjoy the "official" anniversary.
(Since you had some interest in going on the anniversary, though, I am going to guess that you wouldn't start your trip the day after the 17th anyway.

)