What is Nisfu Sha'baan, and should we fast that day?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

What is meant by the middle of Sha'ban is the 15th of the month of Sha'ban, the eighth month of the Islamic calendar.

Fasting during this day is recommended being one of the days of the full moon (the 13th, 14th, and the 15th days of each month). Encouraging us to fast these days, the Prophet said: “If one among you is to fast for three days from every month, let him fast on the three days of the full moon.” [Ahmad and classified by Al-Arna'oot as acceptable]

As for the narration reported by Ibn Maajah that ‘Ali said: “When the middle of Sha'ban comes, then observe additional prayers during its night and fast during its day.”; it is ruled by scholars as weak. Shaykh Al-Albaani classified it as fabricated.

Allah knows best.