The event will be live streamed and questions will be taken over the web, so anyone can join. Facebook will be selecting questions to ask during the event and you can submit questions now by posting them on the facebook event's wall. Get started at the White House's facebook page.
Here are the details:
When: April 20th @ 1:45 PDT / 4:45 EDT
Where: Facebook HQ in Palo Alto
The event will be live streamed at both WhiteHouse.gov/live and facebook.com/ObamaWhiteHouse
To submit a question without a facebook account, use this form.