Customers at hundreds of Starbucks Corp. stores can expect a shot of labor activism along with their espresso this morning.

Employees at the coffee giant’s unionized stores plan to walk off the job on Thursday on one of the busiest sales days of the year — Red Cup Day, when Starbucks gives out holiday-themed reusable cups. Starbucks Workers United, which has been locked in a bitter battle with management over its efforts to organize, is calling the action its biggest strike ever and is asking that customers join in rather than buy their usual latte or cold brew.