Cinco de Mayo (fifth of May in Spanish) is a holiday commemorating the victory of Mexican's army in 1962 during the Franco-Mexican War. While it's not widely celebrated in Mexico, it's become a worldwide celebration of Mexican culture and heritage, synonymous with traditional mariachi music and street festivals.
It's not the best known holiday in London, but it should be. It gives you a fantastic reason to eat guacamole and tacos and drink as much tequila as you can. Just don't wear a sombrero - it's cultural appropriation.
While the 5th of May isn't until next Tuesday, the bank holiday weekend is the perfect opportunity to celebrate.