May I fast the white days (the 13th, 14th, and 15th of every lunar month) separately? For example, can I fast the 1st, 15th, and 30th of every month instead? Or do I have to fast the three days as the 13th, 14th, and 15th of every lunar month?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

It is better for you to fast on the white days—the 13th, 14th, and 15th of every lunar month. This goes in line with the Ahadeeth that have been quoted in this regard. Abu Tharr narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said to him: “O Abu Tharr! If you are going to fast three days in a month, then fast the 13th, 14th, and 15th thereof.” [At-Tirmithi; Al-Albaani: good-authentic]

That being said, if you fast the 1st, 15th, and 30th of every month, then this does not count as the fasting of the white days; only the 15th will count. Nevertheless, you shall attain the reward of voluntary fasting, Allaah willing. The Prophet, sallallaahu ‘alayhi wa sallam, said: “Whoever fasts a day for the sake of Allaah, Allaah will distant his face seventy years away from Hellfire.” [Al-Bukhari and Muslim; this is Al-Bukhari's wording]  

The Muslim attains the reward of three days every month if he fasts any three days thereof, whether consecutively or separately, and whether at the beginning or at the end of the month. Mu‘aathah said that he asked ‘Aa’ishah: “Did the Prophet, sallallaahu ‘alayhi wa sallam, fast three days of each month?” ‘Aa’ishah said, “Yes!” Mu‘athah also asked ‘Aa’ishah, “What days did he observe?” ‘Aa’ishah said, “He would not care what days of the month he would fast.” [Abu Daawood]

However, the Muslim is more recommended to fast the three white days as defined in the above-mentioned Hadeeth.

Allaah Knows best.