If you are watching the later seasons, here is a quick guide to their respective 9th episodes:
Fans have noted that in the light novel, Celty's head is described as having hair "as black as darkness," whereas the anime depicts it with brown hair. Durarara!! Episode 9
This episode is often cited by fans as the point where the story "connects the dots" and the main plot finally gains momentum. Other "Episode 9" Variants If you are watching the later seasons, here