The Mentalist Season 2 Episode 23 (The Mentalist s02e23) with title “Red Sky in the Morning” airs on Thursday, May 20th 2010 at 10:00 pm via CBS.
Here is summary for The Mentalist Season 2 Episode 23 :

In the second-season finale, Jane and also Kristina’s first date ends abruptly when a murderer copying Red John’s MO strikes. When Kristina tries to contact the real Red John, her life and Jane’s are threatened.

Few hours after original series aired on TV, online streaming video for The Mentalist Season 2 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.