https://en.al-dirassa.com/why-we-fast-on-the-day-of-ashura/

The day of the Ashura is the 10th day of the month of al-Muharram, which is the first month of the Hegira calendar and one of the four sacred months of Allah.
 
In this regard, Allah the Almighty says (close translation):
 
Indeed, the number of months with Allah is twelve [lunar] months in the register of Allah [from] the day He created the heavens and the earth; of these, four are sacred. That is the correct religion, so do not wrong yourselves during them. And fight against the disbelievers collectively as they fight against you collectively. And know that Allah is with the righteous [who fear Him].” (Quran, 9:36).