1- Six days in the month of Shawwal (after the month of Ramadan)

The Prophet ﷺsaid “Whosoever fasts the month of Ramadan and thereafter follows it up with six days of fasting in Shawwal, it will be recorded (altogether) for him as a complete year of fasting.” [ Source: Muslim.]
It is the same if he does it consecutively or spreads it over the entire month.

2- First nine days of Dhul Hijjah (the twelfth month of the Islamic calendar)

The Prophet ﷺ said : “There is no other day in which good deeds performed are so pleasing to Allah than these days (i.e. the first ten days of the twelfth month). The companions inquired: “What about jihad (fighting) in the cause of Allah?” The Prophet said: “Not even jihad in the cause of Allah (is up to its rewards), except for a man who went out in the cause of Allah with his life and wealth and did not return with anything of either (of them).” [ Source: Bukhari.]

3- Fasting on ‘Ashura

‘Ashura (10th day of Muharram)This is the tenth day of Muharram, the month of Allah Almighty.
This is because the Prophet said: “And as for fasting on the day of ‘Ashura, I expect that Allah Almighty shall forgive the sins of the year prior to it.” [ Source: Muslim.]
source: https://www.al-feqh.com/en/voluntary...gatory-fasting