Halloween sales represent nearly 25 percent of Party City's total retail revenue, and nearly 20 percent of its total revenue, Harrison told CNBC's "Squawk Box."
He said that where the chain goes beyond Amazon is in the opportunities consumers have at Party City to buy separate costume pieces and put them together more creatively, rather than simply ordering something preset online.
"We take something thematically and really blow it out with multiple options," Harrison said.
For example, "you could have five or six people go to a party and they all look entirely different although they're all [dressed as] Spider Girl," he said.
The CEO said that other major occasions that give Party City a comparable boost in business are graduation and summer parties.