Is there any religious requirement to fast on holy nights (Kandil)?

The Prophet Muhammad (peace be upon him) said, “When the fifteenth night of Sha’ban (that is, the Night of Bara’ah) comes, spend that night in worship and fast during its day. Allah manifests Himself (with His mercy) to the lowest heaven at sunset and, until dawn, says: ‘Is there anyone seeking forgiveness so that I may forgive them? Is there anyone seeking provision so that I may provide for them? Is there anyone afflicted so that I may grant them well-being? Is there anyone who asks…’” (Ibn Majah, Iqamat al-Salawat, 191 [1388]; see also Tirmidhi, Sawm, 39 [739]). On the other hand, the Prophet (peace be upon him) used to fast during the first nine days of Dhu’l-Hijjah (Abu Dawud, Sawm, 61 [2437]; Nasa’i, Sawm, 70 [2372]), on Mondays and Thursdays, on the days of Ashura and Arafah (Muslim, Sawm, 196-197 [1162]; Nasa’i, Sawm, 70 [2372]; Ibn Majah, Sawm, 41-42 [1733, 1739-1740]), and when asked about fasting on Mondays, he replied, “This is the day I was born and the day I was sent as a Prophet—or the day the Qur’an was revealed to me.” (Muslim, Sawm, 197-198 [1162]).

Based on these and similar reports, some Islamic scholars have stated that it is recommended (mustahabb) to revive other virtuous days and nights with acts of worship.

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu