Once a month you will need to prune or delete out of date loan reservations and advanced bookings.  Reservations are 'valid' for a period of one month.

Once the Expiry date has passed there is no longer any need to keep these in the system.  To prune old reservations go into Utilities / Update / Circulation / Prune Reservations.  Set the date that you would like to use and click OK.

(More detail necessary)