al-Layl 92:2 commentary

92/2

وَٱلنَّهَارِ إِذَا تَجَلَّىٰ

Wa'l-nahāri idhā tajallā "And by the day when it comes out."

تَجَلَّى — to open oneself

The root j-l-w was treated at al-Shams 91/3: to polish, to lift what lies over a thing and make the real surface visible.

The difference is in the form. In al-Shams the verb was in Form II (jallā — it displayed it): transitive, doing something to something. Here it is in Form V (tajallā): reflexive, happening of itself. "It showed itself, it came out, it appeared."

The Qurʾān uses the same form in one of its weightiest places: "When his Lord manifested Himself to the mountain, He made it crumble to dust" (al-Aʿrāf 7/143).

Its use for the day sets up this picture: the day does not come, it opens. Not as a curtain is drawn back, but as a thing itself becomes visible.

Why is the order reversed? — day first in al-Shams, night first in al-Layl

This is the most visible difference between the two sūras, and it is worth dwelling on.

al-Shams 91/3-4: wa'l-nahāri… wa'l-layli — first the day, then the night.
al-Layl 92/1-2: wa'l-layli… wa'l-nahāri — first the night, then the day.

The same pair, in reverse order. The explanations offered are these:

Explanation	How it works	Its force
The name and opening of the sūra	Every sūra begins from the element that carries its name. Al-Shams opens with the sun; the day, being the sun's work, comes adjoining it. Al-Layl opens directly with the night.	Simple and correct; but it answers only the "how", not the "why"
The internal logic of the series	In al-Shams the series begins from the source of light (sun → moon → day → night): from light to darkness. In al-Layl the reverse: from darkness to light.	It fits the text
The order of creation	In the Qurʾān darkness is named before light: "He made the darknesses and the light" (al-Anʿām 6/1). Al-Layl follows this order.	Supported from within the Qurʾān, but it does not explain al-Shams's order

To these I want to add a fourth, and I state that it is my own reading:

The order of the oaths in each of the two sūras gives in advance the direction of the whole of that sūra.

Al-Shams begins with light and continues with darkness — and the sūra ends with the damdama that comes down upon a people, with a covering-over. From light to darkness.
Al-Layl begins with darkness and continues with light — and the sūra ends with "and he will surely be well pleased" (92/21), with an opening. From darkness to light.

The order of the oaths, that is, is a small-scale model of the direction in which the sūras run.

A qualification has to be entered on this reading: in the middle of al-Layl (5-10) the order runs from good to bad, so the matter lies not in every duality but in the general arc of the sūra. The sūra's last movement is between verses 14 and 21, and there the order is clear: first the fire and al-ashqā (14-16), then al-atqā and contentment (17-21). The sūra closes moving from darkness to light.