Community Season 1 Episode 25 (Community s01e25) with title “Pascal’s Triangle Revisited” airs on Thursday, May 20th 2010 at 08:00 pm via NBC.
Here is summary for Community Season 1 Episode 25 :

In the Season 1 finale, Britta and also Professor Slater vie for Jeff’s affections. Elsewhere, Abed doesn’t ask Troy to move in with him, leaving Troy feeling confused about their friendship.

Few hours after original series aired on TV, online streaming video for Community Season 1 Episode 25 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.