ما هي الأيام الواجب صيامها في شهر محرم؟

It is not obligatory to fast the month of Muharram or any of its days. Rather, scholars differed on whether fasting the day of Ashura was obligatory before Ramadan was enjoined. However, after Ramadan was enjoined, fasting Ashura became mustahabb (recommended) by consensus. The recommendation of fasting Muharram is evidenced by the Prophet’s (peace and blessings be upon him) saying: "The best fasting after Ramadan is Allah’s month of Muharram." It is recommended to fast Ashura along with Tasu'a (the ninth of Muharram), or with a day before it and a day after it, or to suffice with it and a day before or after it, in order to differ from the Jews. Fasting it alone is sufficient.