The story
For Sunni Muslims the day is a fast. The Prophet found the Jews of Medina fasting on it in memory of the parting of the sea before Moses, and said that Muslims had a better claim to Moses than they did — so the fast was kept, and remains a recommended one.
For Shīʿa Muslims the day is the death of Ḥusayn, the Prophet's grandson, at Karbalā in 680. He refused allegiance to the caliph Yazīd, rode out with about seventy men and his family, was surrounded, cut off from the Euphrates for three days, and killed. His half-brother ʿAbbās died trying to bring water to the children.
The two observances share a date and almost nothing else. One is a quiet fast; the other is ten days of public mourning that end in the streets.
