On Fasting: Is there voluntary fasting in it? Yes. And among it is: fasting three days of every month, which the Prophet (peace and blessings be upon him) recommended to three of his Companions. He recommended it to Abu Hurayrah, Abu Dharr, and Abu al-Darda (may Allah be pleased with them) that they fast three days of every month.

The Shaykh : Is there voluntary fasting in fasting? Yes. And among it is: fasting three days of every month, which the Prophet (peace and blessings be upon him) recommended to three of his Companions. He recommended it to Abu Hurayrah, Abu Dharr, and Abu al-Darda (may Allah be pleased with them) that they fast three days of every month. And the Prophet (peace and blessings be upon him) said regarding it: . Aisha said: . And that is the case. If you fast three days in the first ten, it is valid; in the middle ten, it is valid; in the last ten, it is valid. But the best is that they be the white days, which are the thirteenth, the fourteenth, and the fifteenth. This is better than them being in the other days. However, the Sunnah is achieved even if they are in days other than these, just as if you prayed at the beginning of its time, it would be better, but if you prayed it after that, it would still be a prayer within its time. These three days, with regard to the month, the whole month is a time for them, but the best is that they be in these specific days. So if a person leaves these specific days for a legitimate Shari'ah interest, such as honoring a guest who has come to him, for example, then leaving them and fasting them on another day, on other days, is better than fasting them on these three days.
And that is in order to take into account the interests in acts of worship.