Timeline

It appears that this series may take place between episodes 3 and 4 or 4 and 5 of the movies. Is anyone sure where this series fits in? Since Darth Vader already exists, it has to be after episode 3. Luke is mentioned so it could be between 4 and 5 or 5 and 6. It also could be taking place during episodes 4, 5 or 6. Luke wasn't a galaxy wide known character until after episode 4, so it is most likely to be between episodes 4 and 5 or 5 and 6. Any help nailing this down would be greatly appreciated.