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)
أَمَرَنَا رَسُولُ اللهِ ﷺ أَنْ نَصُومَ ثَلَاثَ عَشْرَةَ وَأَرْبَعَ عَشْرَةَ وَخَمْسَ عَشْرَةَ