The idea is to commemorate the martyrdom of Hussein ibn Ali, grandson of Mohammad, whose death marked the split between the Sunni and Shia sects.
Ashura is celebrated by publicly beating, cutting, and/or flagellating yourself. Children are expected to participate. It’s a quaint multicultural practice that we’ll be seeing more of as Islam supplants Christianity in the West.
Just like they have been doing since Allah was called Marduk over 3000 years ago.