The Messenger of Allah ﷺ used to command us to fast the days of the white (nights): thirteenth, fourteenth and fifteenth of the month. He said: This is like keeping perpetual fast. (Using translation from Abū Dāʾūd 2449)
كَانَ رَسُولُ اللهِ ﷺ يَأْمُرُ بِصِيَامِ لَيَالِي الْبِيضِ ثَلَاثَ عَشْرَةَ وَأَرْبَعَ عَشْرَةَ وَخَمْسَ عَشْرَةَ وَقَالَ هِيَ كَصَوْمِ الدَّهْرِ