Workers United announced on social media platform X that hundreds of Starbucks (SBUX.O) employees had walked out of work during a significant promotional event on Thursday in protest of inadequate staffing and schedules.

The protest coincides with Starbucks’ Red Cup Day promotion, in which patrons who purchase coffee are given free, reusable, red cups with holiday designs.

More than half of Starbucks’ US locations were open and “serving customers” this morning, the company claimed on Thursday. However, it added that “a few dozen stores with some partners (were) on strike.”

About a dozen workers picketed outside Starbucks’ Astor Place outlet at the New York University’s campus chanting “no contract, no coffee” and other rhymes. Meanwhile, the Astor Place continued to fill with NYU staff and students placing orders.

Red Cup day has typically been a major driver of store traffic, with Placer.ai data showing that visits to U.S. Starbucks stores on the day last year jumped 94% over the daily average for the full year.