you mentioned in "best day ever" review how muffy is the one who speaks the episode's title card, even though she's not in the episode at all. that's a trend i've noticed in quite a few episodes-- the character who announces the title card isn't in the episode a whole lot, or at least isn't a major character in it. and another thing I noticed-- arthur himself never announces the title cards!
In regards to the last day episode, I've seen it and it is about the last day of 3rd grade/preschool. Without giving away too much, it doesn't exactly feel like a finale, but until a new season is confirmed, I just can't be sure! I hope they do continue on into the 4th grade.