Melissa Shield sends an e-mail to her "Uncle Might" and he receives it after finishing up a special class with his students. School in Japan is on summer break and Melissa wants All Might to surprise her father during the I-Expo.
Melissa gives All Might two formal invitations to the expo. All Might finds Izuku after school that day and asks him to come to I-Island. He wants Izuku to get this experience to help better him as a hero. Izuku knows a lot about the island and is more than happy to join. Although he's surprised when he learns they need to fly there right away. 
Early Access Event
Scientists, heroes, and people from all around the world gather on I-Island to showcase the advancement of Quirk technology. This event is sponsored by big companies that send invites for highly regarded Pro Heroes. Civilians and students can also receive invitations through Pros.
Invitations give participants early access to dozens of vendors using Quirks as entertainment. This includes access to the Pavilion, cafes, musical shows and much more. These vendors even offer temporary jobs for students. Early access to the I-Expo contains the popular "Villain Attack" game where competitors race against the clock to defeat robotic villains. Anyone with an invitation can also attend a reception party with guests from across the industry.
During the reception party at the end of the expo's preopening, Wolfram and his henchmen hijack the Central Tower and turns the security system on the heroes.