According to 9to5Mac, citing sources, Apple has informed some employees at its retail stores that they'll need to plan for an "overnight" on July 24. Overnights have become a key indicator that something new is coming to Apple's retail stores. Overnights mean that employees work outside their stores' open hours to prepare their location for the next day's new sale. Generally, that means Macs, iPhones, or iPads -- but not always.
If 9to5Mac's sources are to be believed, this time around it means OS X Mountain Lion.