Answered by Sayyidi Habib Umar bin Hafiz (may Allah protect him and benefit us by him)
Is it recommended to fast on the day after the night of the Isra and Mi`raj (27th Rajab)?
Generally speaking fasting in the sacred months is better than fasting in the other months. There is a sound hadith in which Ibn `Umar was asked whether the Prophet ﷺ used to fast in Rajab. He said: “Yes, and he would ennoble it (meaning the month of Rajab).” Some of the pious recommend fasting on the day of the of the Isra and Mi`raj but it is not something that has been directly narrated on the authority of the Prophet ﷺ . Above all we should seek a true connection with the Prophet, the one who was taken on the Isra and Mi`raj.