Why were the extra employees working late on Saturday night in From the Mixed-Up Files of Mrs. Basil E. Frankweiler?
There are extra employees at the museum because the museum has recently acquired a bargain statue that might be a Michelangelo.
Jamie and Claudia are hiding in the museum because Claudia decided to run away from home. They wonder why there are so many people there, and one day they realize that they are all coming to look at a statue. The statue is special, and as word spreads more and more people come to see the statue.
[Now] they reached what everyone was standing in line to see. A statue of an angel; her arms were folded, and she was looking holy. (Ch. 4)
Since the museum paid so little for the statue and it might have been sculpted by the famous sculptor, the people are coming in droves. The museum needs more staff and later hours to keep up with all of the demand. There is an increase in interest, an increase in attendance, and an increase in security because of it.
The children are just as excited. Claudia feels an affinity to the statue, thinking it looks like a relative. They decide to find out for themselves, providing the book’s main mystery.