Starbucks Korea will simultaneously close all its stores for a mandatory history lesson, after a disastrous promotion that evoked memories of a pro-democracy massacre sparked public and political backlash.

More than 2,000 stores will temporarily close at 3pm on 22 June, the company said, so staff can watch recorded lectures on modern Korean history and engage in “social sensitivity” training. The half-day closures will cost Starbucks an estimated 2.1bn won ($1.4m) in lost sales, according to data firm IGAWorks.

The measures follow a public relations crisis triggered when Starbucks Korea ran a discount promotion for its “Tank” tumbler series on 18 May, the anniversary of a 1980 massacre in Gwangju. The promotion led to store boycotts, customers smashing Starbucks mug and tumblers and government ministries cutting ties with the chain.

Chung Yong-jin, the billionaire chair of Shinsegae Group, which operates Starbucks Korea under licence from its US parent company, will take the same training on 24 June alongside other executives.

The curriculum covers major events in contemporary Korean history and how companies should account for historical and social sensitivities in their marketing decisions.

Shinsegae said the shutdown was intended to demonstrate the seriousness with which it viewed the incident and to prevent a repeat of similar controversies. The only exclusion to the shuttering will be a handful of outlets at airports, a company spokesperson said.

Payment volumes, which plunged 26% in the week after the controversy, have shown signs of partial recovery, rising 12.8% in the first week of June, according to market data, but they remain about 25% below pre-controversy levels.

The Gwangju massacre is a painful memory for many. Over 10 violent days, paratroopers crushed pro-democracy protests against military strongman Chun Doo-hwan. Victims’ groups say hundreds were killed.

Starbucks branded the date of its promotion “Tank Day”. It also featured the slogan “thwack on the desk”, evoking a notorious police explanation for the 1987 torture death of student activist Park Jong-chul. Authorities falsely claimed he had died after an officer “hit the desk with a thwack” during questioning.

Marketers chose the “thwack” slogan after consulting an AI tool for suggestions, Shinsegae Group said. It turned out some managers who approved the campaign never opened the email attachments showing the marketing material.

The company pulled the campaign within hours, but the fallout was swift and the chief executive was sacked the same day.

Starbucks said it was “deeply sorry for an unacceptable marketing incident” and that it “should never have happened”. Chung issued a written apology, and also apologised in a televised press conference where he bowed three times.

Starbucks’ Seattle headquarters sent a written apology directly to the May 18 Foundation, one of the main bodies representing Gwangju victims, after the foundation wrote to the company demanding a formal response.

An internal investigation found no evidence of deliberate intent, though a police investigation is ongoing. Chung and the former chief executive have been registered as criminal suspects by Seoul police.

Attitudes towards the Gwangju Uprising remain one of the deepest fault lines in South Korean society.

Far-right groups have kept alive a decades-old, discredited state narrative that the Gwangju protesters were North Korean sympathisers, a claim the supreme court ruled false and defamatory earlier this year.