There was a 5th anniversary celebration and a 10th anniversary celebration, and during those events there were a number of limited-time only paintings. I would really like an opportunity in November to acquire them if possible.
Lots of stuff happening this fall. New expansion and Anniversary are the big ones. I would hope that they do something fun.
best celebration for 15th anniversary is new, very long heritage quest, beginning from egg (earlier than 19th) and ending in chaos , with awards in each tier (longer than longest already available)