التفسير التبليغي
(And [by] ten nights) (89:2). The ten nights are left indefinite (nakkar) because indefiniteness (tankir) in this context serves to magnify them, for indefiniteness can be for magnification.
(And [by] ten nights) refers to the nights of the first ten days of the month of Dhul-Hijjah, which Allah has honored so that people may hasten to do good deeds.
(And [by] ten nights) These specific ten nights are distinguished from other ten-night periods by a virtue not found in others.
A time period that encompasses great deeds is worthy for the Lord – Exalted and Glorified be He – to swear by it.
This is an admonition regarding the lesson contained in the number, in terms of what regulates the life of a Muslim.
Source: التفسير التبليغي