Revealing mistake: When Sean shoots at Dennis the Bookseller's feet, puffs of sand do not erupt from the ground.
Continuity mistake: Near the beginning, a woman goes into an antique bookshop and hands the man who works there a battered book. At first, the book is the right way up, but when he puts it down it's suddenly upside down. When we see the book again, now it's the right way up.
Answer: The movie never clearly says. It was some event that required tickets, as the mom runs back into the house to get them. And it was something Sally really wanted to go to, but it is never clear what the event was.