Unless you're there early, I expect the City Hall garage (across from the park) will be difficulat to park in. So will a lot of the close by street parking. There are other public parking garages within a couple of blocks. See map at:
http://www.cityofsacramento.org/transportation/dot_media/parking_media/pdf/Off_Map.pdf
(Click on the blue tag for the garage and you'll see rates and hours. Note: they may change rates for "special events")
If you can use light rail, can get off the LR just a block from the park. Remember, there will be people driving into town to go to this, just as the mass exodus out is going on.
50,000? I'd love to make that kind of statement. I'm taking vacation time off to be there and hope I can get the family to go with me.