Tafsir of Al-Mursalat 77:34-35

"Woe that Day to the deniers! This is a Day on which they shall not speak."

The reference is to the time of their entry into the Fire; that is, this is a Day on which they shall not utter anything due to the enormity of the shock and the excess of bewilderment. This does not contradict what is mentioned in another place regarding them speaking, for the Day of Resurrection is long and has various stations and timings; in some, they speak, and in others, they do not. It is also permitted that the meaning is: "This is a Day on which they shall not speak anything that will benefit them," rendering their speech—due to its lack of benefit—as if it were no speech at all.

Al-A'mash, Al-A'raj, Zayd ibn Ali, Isa, Abu Haywah, and Asim (in one narration) read "hādha yawma" (هذا يومَ) with a fatha. This fatha is one of inflection (i'rab), on the basis that "this" (hādha) is an indication of what has been mentioned, and "Day" (yawma) is in the accusative case as an adverb, connected to a deleted predicate that serves as the news for "this." That is: "This which has been mentioned of the threat occurs on a Day on which they shall not speak."

It is also said that it is a fatha of indeclinability (bina'), and "Day" is in the nominative position as the predicate; it is rendered indeclinable because it is annexed to a sentence and due to its resemblance to what is indeclinable. The author of Al-Lawamih reported that Isa said the indeclinability of "Day" on fatha when paired with "la" (lā) is a lower dialect of Mudar, for they treated it along with "la" as a single noun.

You know that the Basrans do not permit, in any way, the indeclinability of an adverb annexed to a sentence starting with an affirmative or negative present-tense verb, and that what was mentioned is your school of thought.