What is the ruling on fasting a single day of the White Days?

The Sunnah is to fast three days every month, and it is preferable that these be the White Days (13th, 14th, 15th). If one is unable to fast the White Days and fasts other days, it is sufficient. It is permissible to fast the three days consecutively or separately, whether at the beginning of the month, in the middle, or at the end. If one is unable to fast all of the White Days but fasts some of them and completes the three from other days, that is good. An exception to this is the thirteenth of Dhul-Hijjah, as it is one of the Tashriq Days, which are forbidden to fast. One can then fast the two days after it. It is permissible to fast on a Saturday or Sunday if they coincide with the White Days, due to the virtue of the day, not for the purpose of specification.