Unionized Starbucks Corp. baristas plan to hold their biggest strike yet this week, accusing the coffee giant of refusing to fairly negotiate at cafes that voted to organize.

Thousands of employees at hundreds of sites will mount one-day work stoppages on Thursday, according to the union Starbucks Workers United. The strike is pegged to the company’s Red Cup Day, a popular promotional event when Starbucks gives out holiday-themed reusable cups. The union says Starbucks has illegally refused to negotiate in good faith over issues including staffing and scheduling that are particularly onerous during such promotions.