Short answer
Apart from Ramadan, the established sunnah fasts are the Day of Arafah, Ashura with a day next to it, six days of Shawwal, the white days (13th to 15th of each month), Mondays and Thursdays, and extra fasting in Muharram and Sha'ban. The best voluntary fast is the fast of Dawud: one day on, one day off.
Fasts other than Ramadan, vows and make-ups are nafl (voluntary). The emphasised ones are Ashura with the 9th, the Day of Arafah for those not on Hajj, six days of Shawwal, three days each month (best on the white days), Mondays and Thursdays, and the first nine days of Dhul Hijjah. Fasting every day of the year without a break is makruh.
Other madhhabs
- Maliki
- Imam Malik disliked fasting the six days of Shawwal straight after Eid where people might think they are part of Ramadan; later Maliki scholars allow them when that worry is absent.
Explanation
Fasting is the one deed that Allah calls His own: every deed of the son of Adam is for him except fasting. Voluntary fasts let you keep that closeness all year. Here they are, from the most emphasised to the general.
- The Day of Arafah (9 Dhul Hijjah), for those not on Hajj. It expiates the sins of the year before and the year after. See fasting on Arafah.
- Ashura (10 Muharram), with the 9th or the 11th. It expiates the year before. See fasting only the 10th.
- Six days of Shawwal, which with Ramadan are like fasting the whole year. See the six days of Shawwal.
- The white days and Mondays and Thursdays. See Mondays, Thursdays and the white days.
- The month of Muharram, the best month for fasting after Ramadan, and Sha'ban, in which the Prophet ﷺ fasted more than in any month besides Ramadan.
- The first nine days of Dhul Hijjah. See the first ten days of Dhul Hijjah.
The best day to fast. The Prophet ﷺ said the fasting most beloved to Allah is the fasting of Dawud, who fasted one day and left one day. That is the most a person should aim for. Fasting every day without a break is disliked, and some days must not be fasted at all: see fasting on Eid.
These fasts are rewarded but not required. In the Hanafi school you may make the intention for a voluntary fast in the morning, before midday, as long as you have not eaten. Missed Ramadan fasts are a duty and come before the extras in importance.
The evidence
Quran, Surah Al-Ahzaab 33:35
إِنَّ ٱلْمُسْلِمِينَ وَٱلْمُسْلِمَٰتِ وَٱلْمُؤْمِنِينَ وَٱلْمُؤْمِنَٰتِ وَٱلْقَٰنِتِينَ وَٱلْقَٰنِتَٰتِ وَٱلصَّٰدِقِينَ وَٱلصَّٰدِقَٰتِ وَٱلصَّٰبِرِينَ وَٱلصَّٰبِرَٰتِ وَٱلْخَٰشِعِينَ وَٱلْخَٰشِعَٰتِ وَٱلْمُتَصَدِّقِينَ وَٱلْمُتَصَدِّقَٰتِ وَٱلصَّٰٓئِمِينَ وَٱلصَّٰٓئِمَٰتِ وَٱلْحَٰفِظِينَ فُرُوجَهُمْ وَٱلْحَٰفِظَٰتِ وَٱلذَّٰكِرِينَ ٱللَّهَ كَثِيرًۭا وَٱلذَّٰكِرَٰتِ أَعَدَّ ٱللَّهُ لَهُم مَّغْفِرَةًۭ وَأَجْرًا عَظِيمًۭا ﴿٣٥﴾
Indeed, the Muslim men and Muslim women, the believing men and believing women, the obedient men and obedient women, the truthful men and truthful women, the patient men and patient women, the humble men and humble women, the charitable men and charitable women, the fasting men and fasting women, the men who guard their private parts and the women who do so, and the men who remember Allah often and the women who do so - for them Allah has prepared forgiveness and a great reward.
Allah praises those who fast and promises them forgiveness and a great reward.
Sahih al-Bukhari 1131
Sahih... وأحب الصيام إلى الله صيام داود ...
"The most beloved fasting to Allah is the fasting of Dawud."
The best pattern of voluntary fasting is one day on and one day off.
Sahih Muslim 1163
Sahih... أفضل الصيام بعد رمضان شهر الله المحرم ...
"The best fasting after Ramadan is in Allah's month, Muharram."
Fasting in Muharram has special merit.
Where the ruling is recorded
- al-Durr al-Mukhtar, Kitab al-Sawm, opening section on the kinds of fasting. Fasts are fard, wajib, masnun, mandub, nafl or makruh, with examples of each.
- Nur al-Idah with Maraqi al-Falah, Kitab al-Sawm, opening chapter. The recommended fasts: three days each month, Mondays and Thursdays, six of Shawwal, Arafah and Ashura.
- Mukhtasar Khalil with al-Sharh al-Kabir, Bab al-Siyam. The Maliki dislike of fasting six days of Shawwal joined to Ramadan.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.