وَٱلْيَوْمِ ٱلْمَوْعُودِ
Root: و-ع-د. To give one's word, to announce that something will happen. The Qurʾān uses this root for both good and bad tidings; for bad tidings there is in addition the form إِيعَاد / وَعِيد (waʿīd — threat).
مَوْعُود is a passive participle: "that which has been promised". The day, that is, is described as being itself a matter of a given word.
The choice of this word is meaningful. The verse does not say "the day of resurrection" or "the day of reckoning". It says "the day of which the word was given" — that is, what defines the day is not what will happen in it but the word given about it.
The majority of the exegetes explain this as the day of resurrection, and the context supports it: the sūra is a sūra of reckoning.
The whole tension of the sūra is this: the wrongdoers were not punished in this world, the wronged were not delivered in this world. Where, then, is the verdict?
The second verse is the name of that "where". Already in its first oaths the sūra says on what ground the reckoning will be held.
And one thing more: the word mawʿūd points to a word given. The last two verses of the sūra will also point to a word (to the Qurʾān). The sūra, that is, opens with a word and closes with a word.