The Virtues of al-Muḥarram
عَنْ أَبِي هُرَيْرَةَ رَضِيَ اللهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللهِ ﷺ: (( أَفْضَلُ الصِّيَامِ بَعْدَ رَمَضَانَ شَهْرُ اللهِ الْمُحَرَّمُ، وَأَفْضَلُ الصَّلَاةِ بَعْدَ الْفَرِيْضَةِ صَلَاةُ اللَّيْلِ )) . رَوَاهُ مُسْلِمٌ
On the authority of Abū Hurayrah, may Allāh be pleased with him, who said, “The Messenger of Allāh ﷺ said, “The best fast after Ramaḍān is the month of Allāh al-Muḥarram, and the best prayer after the obligatory is the night prayer.”” Reported by Muslim (1163).
al-Muḥarram is the Best Month of Optional Fasting
The fasting of the month of Ramaḍān is of course one of the pillars of Islām and it is obligatory by consensus of the Muslim Ummah. This ḥadīth indicates that the month of al-Muḥarram, which is the first month of the Islāmic year, is the best of months for voluntary fasts. Consequently, it is highly recommended for one to fast as much as possible during the month of al-Muḥarram because it is the best fast after the obligatory fast. [Sh. ʿUthaymīn in Sharḥ Riyāḍ al-Ṣāliḥīn, 5:203].
When Does al-Muḥarram 1441H Begin?
The month of al-Muḥarram falls between Dhūl-Ḥijjah and Ṣafar. The first of al-Muḥarram might start from this Friday after Maghrib on the 30th of August 2019 or the next Maghrib on Saturday, 31st August 2019.
Fasting on the Day of ʿĀshūrāʾ
As you can see from the ḥadīth above, the Prophet ﷺ strongly recommended fasting during the month of al-Muḥarram. This recommendation is even more emphatic when it comes to the 10th of al-Muḥarram which is the Day of ʿĀshūrāʾ. When the Prophet ﷺ was asked about fasting on the Day of ʿĀshūrāʾ, he said, as recorded by Imām Muslim, “It expiates the sins of the previous year.”
The Background to Fasting the Day of ʿĀshūrāʾ
The fasting of the Day of ʿĀshūrāʾ has ancient origins which predate our Prophet’s risālah. In fact, Imām Muslim records a number of traditions in his Ṣaḥīḥ (see no. 1125 or 2637 onwards]. We learn that the fasting of the Day of ʿĀshūrāʾ was something the Quraysh of al-Jāhiliyyah engaged in and which the Prophet ﷺ also did before the obligation of fasting Ramaḍān.
When the Prophet ﷺ emigrated to al-Madīnah, he found that the Jews were venerating this day, considering it as an Eid for themselves to the extent their women would dress up for the occasion and they would all fast this day as well, and so he asked them why. They replied that this was the day Allāh gave victory to Prophet Mūsá, peace be upon him, and Banī Isrāʾīl over Firʿawn by drowning him and his cohorts, so Mūsá, peace be upon him, would fast that day out of gratitude towards Allāh and they would fast it out of reverence for it as well. The Prophet ﷺ subsequently commanded his Companions to fast that day and said to the Jews, “We are more entitled to Mūsá than you.” However, when Ramaḍān was obligated in the second year of Hijrah, the Prophet ﷺ said regarding fasting the Day of ʿĀshūrāʾ, “Whoever wishes, he can fast it, and whoever wishes, he can leave it.”
Although, he ﷺ left it up to us to decide whether to fast it or not, he still encouraged its fasting as is evident from the ḥadīth above due to the immense reward it contains. Additionally, the Prophet ﷺ himself did not give up fasting the Day of ʿĀshūrāʾ as indicated by Imām Muslim (see 1129); the Prophet ﷺ said, “This is the Day of ʿĀshūrāʾ, and Allāh did not obligate its fasting for you, but I am fasting. So, whoever amongst you wishes to fast, let him fast and whoever wishes not to fast, let him not fast.”
We find that Ibn al-Qayyim (d. 751) refers to this in Zād al-Maʿād (2:61), that the Prophet ﷺ would seek out the Day of ʿĀshūrāʾ in order to fast it out of all the other days. When he came to al-Madīnah, he found the Jews fasting on this day and venerating it, so he ﷺ said, “We are more entitled to Mūsá than you.” He then fasted the Day of ʿĀshūrāʾ and commanded his Companions to do the same. When Ramaḍān was obligated afterwards, he said, “Whoever wishes, he can fast it, and whoever wishes, he can leave it.”
Fast Another Day Along with the 10th
Remember, if you intend to fast the Day of ʿĀshūrāʾ, which may fall on Monday, 9th September 2019, you should try and fast the 9th as well or the 11th along with it, or fast all three days 9th to 11th. This is because the Prophet ﷺ wanted to differ from the Jews who only fasted the 10th of al-Muḥarram. He also wanted to distinguish himself from the Christians who also venerated this day. In Ṣaḥīḥ Muslim (1134), the Prophet ﷺ said, “If I live until the following year, I will fast the 9th.” Sadly, he did not live to see the next year. One can also fast the 11th along with the 10th as the Prophet ﷺ said, “Fast the Day of ʿĀshūrāʾ and contradict the Jews; fast a day before it or a day after it.” This particular ḥadīth is recorded by Ibn Khuzaymah, albeit with a weak isnād, but both al-Bayhaqī and al-Ṭaḥāwī record it with an authentic chain [See Ṣaḥīḥ Ibn Khuzaymah ed. al-Aʿẓamī, 3:290, no. 2095].
The Levels of Fasting the Day of ʿĀshūrāʾ
Ibn al-Qayyim (d. 751) mentions that the fast of the Day of ʿĀshūrāʾ is of three levels:  the best form is to fast a day before it and a day after it,  the second best is to fast the 9th along with the 10th and  the next best is to fast the 10th alone. [Zād al-Maʿād, 2:72]. This sequencing of the levels of fasting the Day of ʿĀshūrāʾ is also held by al-Ḥāfiẓ Ibn Ḥajar (d. 852) in al-Fatḥ.
Sh. ʿUthaymīn mentions 4 levels in his Fatāwá (20:35, no. 395),
The scholars say, with regards to the fast of the Day of ʿĀshūrāʾ,  either it is [fasted] alone, or  he fasts with it the 9th, or  he fasts with it the 11th. The fourth form is to fast the three days.
What If You Only Want to Fast Two Days?
Sh. ʿUthaymīn states that if you only want to fast two days, it is best to fast the 9th and 10th.
Is It Makrūh to Only Fast the 10th Alone?
Sh. ʿUthaymīn states that there is no agreement amongst the scholars regarding fasting on the 10th alone being makrūh (disliked). However, he states it is better to combine it with a day before it or a day after it.
What If She Is Menstruating, Can She Make Up the Fast on Another Day?
Sh. ʿUthaymīn explains that voluntary deeds are of two types;  those which have cause  those which do not have a cause. Those voluntary deeds which have a cause cannot be made up because they end when their causes end. So, for example, the two rakʿahs of prayer when entering the masjid (taḥiyyatul-masjid), if a man comes into the masjid and sits down for some time and then wants to pray taḥiyyatul-masjid, it would not be considered as such because that prayer has a particular cause connected to it, so once that cause is gone, so is the legislation of that act. Thus, if the Day of ʿĀshūrāʾ comes and a woman is menstruating, she cannot fast that day, nor can she make it up later because once the Day of ʿĀshūrāʾ is over, the fast which comes as a result of it comes to an end. [Fatāwá, 20:43, no. 399].
If the Best Month of Optional Fasting is al-Muḥarram, Why Did the Prophet Fast Most of Shaʿbān But Not Most of al-Muḥarram?
Imām al-Nawawī (d. 676) provides two answers to this question. Firstly, it is possible that the Prophet ﷺ only came to know of the virtue of al-Muḥarram near the end of his life. Secondly, the Prophet ﷺ might have been prevented from fasting most of al-Muḥarram due to travelling, illness or some other preventative factor. [Sharḥ Ṣaḥīḥ Muslim, 8:296, no. 2747].
 رواه مسلم (١١٦٣).