Flash Forward Season 1 Episode 21 (Flash Forward s01e21) with title “Countdown” airs on Thursday, May 20th 2010 at 08:00 pm via ABC.
Here is summary for Flash Forward Episode 21 :

Demetri must choose between helping Janis and Simon break into NLAP to stop the blackout from occurring, or turning the fugitive Simon in to the FBI. Meanwhile, Lloyd finds the common denominator that can help him solve the complex equation he was working on in his flashforward

Few hours after original series aired on TV, online streaming video for Flash Forward Season 1 Episode 21 usually available on Net and of course, if video available, as soon as possible will show you where you can watch this episode for free