Yosemite Blog is reporting that Yosemite admission will be FREE on September 27 to celebrate National Public Lands Day. Then, on Sunday the 28th they'll be free again to celebrate newly naturalized citizens.
Normal admission is $20 per car for 7 days.
It's $10 per person if you are coming via an environmentally friendly means though. (bike, walk, bus, motorcycle). See Yosemite admission fees.
Other parks are apparently taking part in this as well. Stay tuned for updates.