Is it permissible to fast the Ayyam Al-Beed intermittently—such as fasting the first, fifteenth, and thirtieth of the month—or must they be fasted on their known days?

It is best to fast the White Days of the month, which are the thirteenth, fourteenth, and fifteenth, based on the Prophet's (peace be upon him) saying to Abu Dharr: "If you fast three days in a month, then fast the thirteenth, fourteenth, and fifteenth."

If a Muslim fasts other days, such as the first, fifteenth, and thirtieth, he is not considered to have fasted the full White Days. However, he will still receive the reward for voluntary fasting, based on the Prophet's (peace be upon him) saying: "Whoever fasts one day for the sake of Allah, Allah will distance his face from the Fire by seventy autumns."

The reward for fasting three days of the month is achieved by fasting any three days of it, whether they are consecutive or separate, from the beginning of the month or its end, based on 'Aishah's (may Allah be pleased with her) statement: "He did not care on which days of the month he would fast."

However, the best is to fast the White Days specified in the noble Hadith (definition).