Criminal Minds Season 5 Episode 23 (Criminal Minds s05e23) with title “Our Darkest Hour” airs on Wednesday, May 26th 2010 at 09:00 pm via CBS.
Here is summary for Criminal Minds Season 5 Finale :

A series of rolling blackouts in the city of Los Angeles lures a serial killer out of hiding, giving him renewed opportunities to continue his killing spree

Few hours after original series aired on TV, online streaming video for Criminal Minds Season 5 Episode 23 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.