Amy, Ion and A&E are both having marathons on Friday (A&E's starts at 8:00 am EST, IIRC, and Ion's at 3:00 pm). A&E's running S4 and S5 eps, and Ion's having the last few eps from S5 (last year) and the best of the early eps from S1, which are excellent ones to watch if you're just getting started with the show. I've got my dvr to pick up "LDSK" and "The Fox" at 7:00 pm and 8:00 pm on Ion, which IMO are two of the show's best episodes.
ETA: And definitely try and catch tonight's CBS ep at 9:00 EST. It's one of my favorite eps from the entire series, and you really don't have to know anything about the characters to follow the storyline. But after you learn the backstory and rewatch it, you'll realize that about 75% of the non-crime part of the episode is in the subtext (Reid's abandonment issues over Gideon, his connection with the mentally ill and mistreated, none of which are stated outright but are all obliquely referred to).