Here are some suggestions. Of course, these suggestions can be used anytime, not just for this event.
There is Wolfram|Alpha, the computational search engine. You can get a lot of information there, but not the same level of detail most are used to from Wikipedia.
If you have some money, and iOS, the $4.99 Wiki Offline app has similar functionality.
Ironically, one of the targets of SOPA and PIPA, The Pirate Bay has an unofficial Wikipedia torrent file (English) current as this year. The file includes a copy of WikiTaxi, which is an offline reader for the articles. Make sure you follow the instructions on TPB's page after downloading (but remember, torrents may take some time to download).
ComputerSolving.com has a list of Wikipedia alternatives to get you through the day, including Encyclopedia.com and Citizendium. Notably, Citizendium requires authors to verify their credentials before posting.
It's only one day, so you might just bite the ad-supported bullet and use The Free Dictionary's encyclopedia, which is basically a mirror of Wikipedia (but with ads). When searching within the The Free Dictionary's site, after you receive a "result," you will have to click the “Wikipedia encyclopedia” tab to read Wikipedia's content.
Just remember: it's only 24 hours.