The largest labour action since a drive to unionise Starbucks outlets

started in late last year will take place on Thursday, 

 according to employees at more than 100 U.S. locations.

The protests are timed to take place on Red Cup Day

 when Starbucks offers free reusable cups to customers who purchase holiday beverages

One of the busiest days of the year,

 according to employees. Starbucks refused to disclose

 the estimated number of red cups it will hand out.