Customers who return to Starbucks with the custom cup, or any reusable cup, will receive a 10-cent discount on their order and Starbucks Rewards members can earn 25 stars when they bring back their reusable red cup for future orders.

It is Red Cup Day at coffee giant Starbucks, an unofficial holiday for java fans who enjoy discounts and rewards. Here's how it works: Stop by a participating Starbucks store on Thursday and get a free, 16-ounce holiday-themed reusable red cup when you purchase one of the coffee chain's special seasonal drinks.

"Red Cup Day," a favorite Starbucks holiday tradition that offers customers free reusable cups, takes place this week -- but there's a catch.