It’s Always Sunny in Philadelphia Season 6 Episode 6 (It’s Always Sunny in Philadelphia s06e06) with title “Mac’s Mom Burns Her House Down” aired on Thursday, October 21st 2010 at 10:00 pm via FX Network.
Here is summary for It’s Always Sunny in Philadelphia Season 6 Episode 6 :

After Mac’s mother (Sandy Martin) burns her house down with a cigarette, Mac decides that she should move in with Charlie’s mom (Lynne Marie Stewart). Meanwhile, Frank offers to take care of the ailing Dee because he wants her to take care of him when he’s old.