When should the recommended three days of fasting per month be observed?

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It is recommended that these three days be the White Days (al-Ayyam al-Beed). Abu Dharr narrated that the Messenger of Allah commanded him: 'O Abu Dharr, when you fast from the month, fast the thirteenth, fourteenth, and fifteenth.' This is supported by a report where the Prophet instructed an Arab to fast the thirteenth, fourteenth, and fifteenth if he intended to fast three days of the month, stating it is like the manner of fasting the whole year. These days are called 'White' because their nights are entirely illuminated by the moon, or because Allah repented for Adam on these days, and his record was whitened.