Final answer:
The first sign that showed Percy it was not the same day was the date on a newspaper after exiting the Lotus Casino in 'The Lightning Thief'.
Step-by-step explanation:
The student is referring to a scene from 'The Lightning Thief', the first book in the 'Percy Jackson & the Olympians' series by Rick Riordan. When Percy exits the Lotus Casino, the first sign that it was not the same day they entered is (a) The date on a newspaper.
This discovery shocks them as they realize that time has passed differently for them while they were inside the casino, leading them to understand that they have been there much longer than they thought.