Tafsir of Al-Qamar 54:6

So turn away from them: The Fa (So) denotes causality, and the consequence is the act of turning away or the command for it. The cause is that [their admonition] will not avail, or the knowledge that it will not avail. By "turning away," it is meant either the cessation of fighting—in which case the verse is abrogated—or the abandonment of argument for the sake of striving—in which case it is Muhkam (definitive). The former is the apparent meaning.

The Day the caller calls: This is an adverbial phrase for "they shall emerge," or an object for a verb implied [as in, "Remember the day"]. It is also said: Do not look. It is permissible that it be an adverb for "will not avail" or for [a verb like] "it is established," with whatever is between them being parenthetical. Or it may be an adverb for "the disbeliever says," or for "turn away," meaning: turn away from interceding for them on the Day of Resurrection. Or, it is governed by an implied verb [such as "look towards"], and upon this is the recitation of Al-Hasan: "Turn away from them until the day..." The intent is the continuation of turning away. All these views are, as you can see, [subject to analysis].

The caller is Israfil (peace be upon him). It is also said: Gabriel (peace be upon him), or an angel other than them appointed for that task. It is also permissible that the "calling" is [a metaphorical] command for the return [to life] on that day, similar to the command in [the verse] "Be, and it is," if one holds the view that it is an allegory; in that case, the caller is Allah, the Almighty and Majestic.

The Waw is omitted from yad’u (calls) in pronunciation due to the meeting of two quiescent letters, and in script in conformity with the pronunciation. The Ya is omitted from ad-da’i (the caller) for lightening [the utterance] and treating the Al as analogous to the Tanwin, because it succeeds it, and a thing may be treated based on its opposite just as it is treated based on its parallel.

To a thing unknown: That is, a heinous thing that souls despise because they have no prior acquaintance with anything like it; it is the terror of the Resurrection. The term "unknown" (nukr) is used as a metonym for "heinous" because that which is heinous is, in most cases, strange and unfamiliar. It is also permissible that it comes from "denial" (inkar), the opposite of acknowledgment. Regardless, it is an adjective in the form of fu’l (with two dammahs), which is rare in adjectives. Examples include rawdah unf (a virgin meadow untouched by grazing), rajul shull (a man light-footed in necessity, quick, of good company, pleasant of soul), and sajh (gentle and easy).

Al-Hasan, Ibn Kathir, and Shibl read it as nukr (with the Kaf quiescent), just as they say shughl and shughl, ’usr and ’usr. This is either a quiescence for lightening, or quiescence is the original state while the dammah is for assimilation. Mujahid, Abu Qilabah, Al-Jahdari, and Zayd ibn Ali read it as nukira (a past tense verb in the passive voice), meaning "denied" or "deemed strange."