Listen to this announcement from a school librarian:
'Good morning, students.
Please listen carefully.
Next week is our annual Book Fair.
To prepare, the library will be closing one hour early every day this week, at 3:00 PM instead of 4:00 PM.
This will give our volunteers time to set up.
Also, please remember that all overdue books must be returned by this Friday to avoid fines.
We want everyone to have a clean slate for the fair.
Thank you.'
What is the main purpose of this announcement?