Jae C. Hong, File, Associated Press
FILE - In this Dec. 13, 2011 file photo, FedEx employees sort packages at the FedEx hub at Los Angeles International Airport in Los Angeles. FedEx shipped about 17 million packages on Dec. 12, the busiest day in company history, it said. That one-day volume was up 10 percent from last year’s busiest day, and doubled total shipments on an average night.

LOUISVILLE, Ky. — The holiday rush is reaching its peak at UPS' largest air hub in Louisville, Ky., where streams of packages are sorted and shipped in a synchronized process that puts the package delivery company to the test.

The world's largest shipper expects to deliver more than 120 million parcels around the world during the peak week leading up to Christmas, up 6 percent from a year ago.

The Atlanta-based company expects the pace to reach its pinnacle Thursday, with close to 26 million packages being delivered.

Rival FedEx Corp. says it shipped about 17 million packages on Dec. 12, its busiest day ever.

The shipping season sends signals about the overall economy. But the grueling package handling also carries personal implications for employees logging long hours to make extra cash.