Sorry for the lack of updates. This site's on hiatus until I have the time & inclination to do any new stuff. There's a new Errors and Weird Stuff up in the Articles section to tide you guys over.

Season 13 starts airing on October 12.

The highlight of the season is the two-parter "The Great MacGrady", a Very Special Episode in which Mrs. McGrady is diagnosed with cancer. 

From the WGBH Pressroom:

When Lakewood Elementary's beloved lunch lady Mrs. MacGrady is diagnosed with cancer, Arthur and 
his friends all react differently to the news:Arthur and D.W. rush to her side with heaps of chicken soup and 
stuffed animals, Muffy tries to pretend that nothing in her life has changed, and Francine is so upset she can't 
even bring herself to visit Mrs. MacGrady. In this moving episode, both parents and kids learn how they can 
come together to support Mrs. MacGrady and each other; and Lance Armstrong makes a special appearance 
to show Francine and the Elwood City residents how they can take a stand, or ride a bike, for cancer survivors. 
"The Great MacGrady" will air every day the week of Monday, October 19. 

The other episodes not mentioned in the last update are:

Looking for Bonnie: Famous rock musician Dean Lomax visits Elwood City and calls on George's dad to fix his equally famous guitar, Bonnie. Legend says that Bonnie is magical, and Buster and George are convinced that just one strum will give them special powers. And, during one late-night sleepover, they sneak into George's dad's workshop to find out if it is true.
The Pride of Lakewood :To boost school pride, Arthur and his classmates form the Lakewood Pride Committee, which immediately and fervently adds members, distributes buttons and creates special chants. But Brain and Sue Ellen, feeling pressured, decide that they do not want to join, and are accused of not loving their school. Can they prove that they are proud of Lakewood Elementary even if they are not part of this new group?

The Secret Origin of Supernova:Arthur is dismayed when he learns that an energy drink endorsed by his favourite comic book character is full of sugar, and that it will not give him any special energy or magic powers at all! Are there any superheroes who are not just out to make money, or must Arthur create his own?
Kung Fool:Fern volunteers to help an elderly neighbour, Tony Wu, with his household chores, and then discovers his past life as a kung fu celebrity. Suddenly she imagines herself as his student, and she will soon be a kung fu master herself! But will Fern be disappointed to learn that dish cleaning and laundry folding are not really part of some special martial arts training? Or can she learn a valuable lesson in acceptance and friendship?

The Secret Guardians: When Arthur, Binky and Sue Ellen stumble upon a beautiful and private woodland oasis right by their homes, they vow to tell no-one and keep it as their secret spot. Before long, however, they start to find evidence of human habitation; have they discovered an ancient civilisation? Or has someone known about this special secret place all along?

There should be one more episode I don't have the info for. Also, the episode pairings might not be the same as how I've listed them-I'm just getting the descriptons where I can.

Some of these episodes have aired ahead of schedule on certain stations.

Also, Philip Seymour Hoffman guest stars in "Fernlets by Fern", which was already mentioned in the previous update.

In other news, the Arthur Character Search is over:

Connor Gordon, 11, of Savage Minn., is about to make 
his television debut. As the grand prize winner of the ARTHUR/All Kids Can Character Search, Connor along 
with his character, Lydia Fox, will be featured on the live-action segment of an ARTHUR episode Tuesday, June 
30 on PBS KIDS GO! (check local listings). The segment features Connor, an aspiring artist, meeting 
ARTHUR creator and author Marc Brown. 

In February 2009, ARTHUR, the award-winning PBS KIDS GO! television series, and CVS Caremark All Kids 
Can, a program dedicated to making life easier for kids with disabilities, invited children ages 6-12 to create anew friend for ARTHUR. The search was designed to educate children about the importance of inclusion andhow children of all abilities can play together. It also encouraged parents and children to think about what life is like for someone they know who has a disability. 

Connor went right to the drawing board. Selected out of an overwhelming 8,500 entries, he won top honors for his character Lydia Fox:a 10-year old girl fox who uses a wheelchair and loves to draw and play basketball. 
Producers of the ARTHUR show were so impressed that they already have plans to include Lydia in two additional 
future episodes. 

So, keep a lookout for this new character.

info from http://pressroom.wgbh.org/pages/arth.html