Narrated Qatadah Ibn Malhan al-Qaysi (May Allah be pleased with him):
The Messenger of Allah (peace and blessing be upon him) used to command us to fast the days of the white (nights): thirteenth, fourteenth and fifteenth of the month. He (peace and blessing be upon him) said: This is like keeping perpetual fast.
[Abu Dawud]
Was Salam
Jashim