A reminder to fast the 9th and 10th or 10th and 11th of Muharram!

قال الخطيب الشربيني: الحكمة في كون صوم يوم عرفة يكفر سنتين، وعاشوراء سنة: أنّ عرفة يوم محمدي – يعني أنّه خاص بأمة محمد صلى الله عليه وسلم – وعاشوراء يوم موسوي، ونبينا أفضل الأنبياء، فكان يومه بسنتين.

(مغني المحتاج 183/2)

Al-Khaṭīb al-Shirbīnī mentioned: The wisdom behind the fast of the day of ‘Arafah expiating (the sin of) two years, and the fast of the day of ‘Āshūrā expiating one year is that the day of ‘Arafah is a “Muhammadī day” - meaning it is specifically associated to the ummah of Muhammad ﷺ - and ‘Āshūrā is a “Mūsawī day” (associated to Mūsā عليه السلام), and our Prophet ﷺ is the most virtuous of all the prophets, so his day will expiate the sins of two years (whereas the day of Mūsā - عليه السلام - will expiate the sins of a single year).

Mughnī al-Muhtāj (2/183)