According to the namesake website, International Clash Day, February 7, "celebrates music as a tool for social consciousness and a band that made it sound so damn good, and an iconic record that still changes lives 40 years later."

Also:

"2019 marked the the 40th anniversary of the Clash’s masterpiece, and one of the greatest records ever made, London Calling.

It’s a powerful record that laments the destructive forces our world has set loose, seemingly out of our ability to control. It explores our search for fulfillment through addictions and bottomless appetites, and grapples with the eternal question of whether to stay true to yourself or slide down that well-worn path toward comfort, status, and ease.

Through melody, rhythm, lyrics and attitude, London Calling lays bare a modern world that seems empty and meaningless, and calls on us to wake up and take action. Join us in celebrating London Calling and the Clash’s message of inclusion, multiculturalism, pro-immigration, anti-hate, anti-racism, and anti-fascism. It’s a message that is arguably even more relevant today than it was 40 years ago."

From wikipedia:

The Clash were an English rock band formed in London in 1976 as a key player in the original wave of British punk rock. They have also contributed to the post-punk and new wave movements that emerged in the wake of punk and employed elements of a variety of genres including reggae, dub, funk, ska, and rockabilly. For most of their recording career, the Clash consisted of lead vocalist and rhythm guitarist Joe Strummer, lead guitarist and lead vocalist Mick Jones, bassist Paul Simonon, and drummer Nicky "Topper" Headon. Headon left the group in 1982 and internal friction led to Jones' departure the following year. The group continued with new members, but finally disbanded in early 1986.

The Clash achieved commercial success in the United Kingdom with the release of their self-titled debut album, The Clash, in 1977. Their third album, London Calling, released in the UK in December 1979, earned them popularity in the United States when it was released there the following month. It was declared the best album of the 1980s a decade later by Rolling Stone. In 1982, they reached new heights of success with the release of Combat Rock, which spawned the US top 10 hit "Rock the Casbah", helping the album to achieve a 2× Platinum certification there. A final album, Cut the Crap, was released in 1985.

The Clash's politicized lyrics, musical experimentation, and rebellious attitude had a far-reaching influence on rock, and alternative rock in particular. In January 2003, shortly after the death of Joe Strummer, the band—including original drummer Terry Chimes—were inducted into the Rock and Roll Hall of Fame. In 2004, Rolling Stone ranked the Clash number 28 on its list of the "100 Greatest Artists of All Time".

This tartan was conceived to commemorate the 60th anniversary of the birth of Joe Strummer (John Graham Mellor). The colours reflect those used in the production of audio casette tape.

For the official website of International Clash Day, click the album cover.