يومIbn Fāris treats it as a single word, yawm, 'one of the days', which is then borrowed for … (root ywm)

Al-yawm (اليوم): its measure is from the rising of the sun to its setting, and its plural is al-ayyām (الأيام). And al-yawm also means "the event/occurrence" (al-kawn); one says, "Good is the brother, so-and-so, on the day (fī al-yawm)," meaning: in the event that occurred, when calamity befalls. He said: "Good is the brother of the battle-clash on the direful day (al-yawm al-yamā)" [36]. He intended to derive an adjective from the noun, and its proper form should have been to say "fī al-yawm al-yawmī," but he transposed it, just as they transposed al-qusiyy (القسي, bows), al-aynuq (الأينق, she-camels), and al-aytub (الأيطب). And the Arabs say of a harsh day: "a day possessed of days" (yawm dhū ayyām), and "a day possessed of days-upon-days" (yawm dhū ayāyīm), on account of the persistence of its evil upon its people. And ayyām (أيام) in its underlying morphological pattern is ayawām, but when the Arabs find in a word both a wāw and a yāʾ in the same position, with the first of the two being unvowelled (sākin), they assimilate them and let the yāʾ predominate, whether it comes before or after the wāw — except in irregular words that are transmitted, such as al-futuwwa (الفتوة) and al-hūwa (الهوة).

Al-yawm (اليوم) is well known, and its plural is ayyām (أيام); its origin is ayawām, which was assimilated. Al-Akhfash said, regarding God's saying, "[a mosque] founded upon piety from the first day" (ussisa ʿalā al-taqwā min awwali yawm): he said, [it means] "from the first of the days," just as one says "I met every man," meaning "all the men." And one says "I dealt with him by the day" (muyāwama), just as one says "by the month" (mushāhara). And sometimes hardship is expressed metaphorically by "the day"; one says "a hard day" (yawm ayūm), just as one says "a hard night" (layla laylāʾ). The rajaz poet said (1): "Good is the brother of the battle-clash on the direful day (al-yawm al-yamā)" (2) — and it is a metathesized form of it: the wāw was delayed and the mīm advanced, then the wāw was changed to yāʾ once it became final, just as they said "adlin" (أدل) as the plural of "dalw" (دلو). And Yām and Khārif are two tribes of Yemen. And Yām ibn Nūḥ, peace be upon him [Noah], drowned in the Flood.

The yāʾ, wāw, and mīm: one word, that is al-yawm (اليوم), "the day": one of the days (al-ayyām). Then they use it metaphorically for a great/momentous event, saying "Good is so-and-so on the day (fī al-yawm)" when a calamity befalls. And a verse is cited: — "Good is the brother of the battle-clash on the direful day (al-yawm al-yamā)" ... Some have said that it is a metathesized form, as though it were originally "kāna fī al-yawm," and that the origin of "ayyām" (أيام) is "abwām," but it was assimilated (contracted). As for words in this chapter that exceed three letters, such as al-yarbūʿ (اليربوع), a small creature, and Yabrīn (يبرين), a place-name, and Yamūd (يموود) and Yalamlam (يلملم), which are two places, and al-yarandaj (اليرندج), which is black hides, and the like — the status of the yāʾ at the beginning of such words is like the status of the hamza in quadriliteral and quinquiliteral words, in that both are augmentative letters, and what counts is only what comes after the yāʾ, just as in the chapter on hamza what counts is what comes after it; and this has already been discussed in the introductory chapters of the book. The eminent, blessed Shaykh and Imām, Abū al-Ḥusayn Aḥmad ibn Fāris — may God's mercy be upon him and may He grant him abundant reward — said: We have mentioned what we undertook at the outset of the book to mention, and this constitutes a sound foundation of the language; as for encompassing the entirety of the speech of the Arabs, that is something no one is capable of except God Almighty, or a prophet among His prophets, peace be upon them, by God's revelation, and that is a matter reserved to Him. Praise be to God, first and last, inwardly and outwardly, and blessings and peace be upon His Messenger Muhammad and all his family, the good and the pure. The completion of the writing of this book, the Book of al-Maqāyīs fī al-Lugha [The Standards of Language], has now taken place. I began the critical edition of this book on the evening of the middle of Dhū al-Qaʿda in the year 1365, and I finished it on the morning of the first day of the blessed month of Dhū al-Ḥijja of the year 1370. Praise be to God who guided us to this, for we would not have been guided had God not guided us — ʿAbd al-Salām Muḥammad Hārūn.

Al-yawm (اليوم) is used to express the time from the rising of the sun to its setting. It may also be used to express a period of time, of whatever duration, as God Almighty said: "Indeed, those of you who turned away on the day the two armies met" (Q. Āl ʿImrān 3:155), and "and they will offer submission to God on that Day" (yawmaʾidhin) (Q. al-Naḥl 16:87), and He said: "Spend from what We have provided you before a Day comes" (Q. al-Baqara 2:254), and other such instances. And His saying, Mighty and Exalted: "and remind them of the Days of God" (bi-ayyāmi Allāh) (Q. Ibrāhīm 14:5) — the ascription of the days to God Almighty is an honoring of their significance, on account of what God poured out upon them of His favors during them. And His saying, Mighty and Exalted: "Say, do you indeed disbelieve in Him who created the earth in two days..." (Q. Fuṣṣilat 41:9) — the discussion of its precise meaning belongs to a book other than this one. And yawm (يوم) is combined with idh (إذ), so one says yawmaʾidhin (يومئذ), as in His saying, Mighty and Exalted: "that Day will be a difficult Day" (fa-dhālika yawmaʾidhin yawmun ʿasīr) (Q. al-Muddaththir 74:9). It may sometimes take declension (yuʿrab) and sometimes be indeclinable (yubnā); when indeclinable, it is because of its being annexed (muḍāf) to idh.

Al-yawm (اليوم): well known; its measure is from the rising of the sun to its setting, and its plural is ayyām (أيام) — it is not pluralized otherwise — and its origin is ayawām, which was assimilated; they did not use for it a plural of multiplicity (jamʿ al-kathra). And His saying, Mighty and Exalted: "and remind them of the Days of God" — the meaning is: remind them of God's favors, which He bestowed upon them therein, and of God's vengeances, by which He took vengeance in those days upon Nūḥ, ʿĀd, and Thamūd. Al-Farrāʾ said: its meaning is that he frightened them with what befell ʿĀd and Thamūd and others of punishment, and with the pardon granted to others — and it is in meaning like your saying, "Deal with them with severity and with gentleness." And Mujāhid said, regarding His saying "they do not expect/fear the Days of God": he said, [it means] His favors. And it is related from Ubayy ibn Kaʿb, from the Prophet, God bless him and grant him peace, regarding His saying "and remind them of the Days of God": he said, "His days are His favors." And Shimr said, regarding their expression "yawmāhu": "a day of generosity, and a day of combat"; and "yawmāhu": "a day of prosperity, and a day of misfortune" — so al-yawm here is in the sense of time/fate (al-dahr), i.e., "that is his fate/lot, thus." And ayyām in the underlying morphological pattern is ayawām, but when the Arabs find in a word both a yāʾ and a wāw in one position, with the first of the two being unvowelled, they assimilate one into the other and let the yāʾ predominate, whether it comes before or after the wāw — except in irregular words that are transmitted, such as al-futuwwa and al-hūwa. And Ibn Kaysān was asked concerning ayyām: why did the wāw disappear? He answered: every yāʾ and wāw where one of them precedes the other with a sākin (unvowelled letter) between them, the wāw turns into a yāʾ in that position, and one is assimilated into the other; from this, ayyām's origin is ayawām, and likewise sayyid and mayyit, whose origins are sayawid and mayawit; most speech follows this pattern except for two words, ṣayyūb and ḥayawa, and had they applied the rule to them they would have said ṣayyib and ḥayya. As for the wāw when it precedes, [consider] your saying "lawaytuhu layyā" and "shawaytuhu shayyā," whose origins are shawayā and lawayā. And Abū al-ʿAbbās Aḥmad ibn Yaḥyā was asked about the Arabs' expression "al-yawma al-yawma," and he said: they mean "al-yawma al-yawma," then they lightened the wāw and said "al-yawma al-yawma," and they say, "Today I shall do such-and-such," not meaning a specific day but meaning the present time; Sībawayh related this. And from this is His saying, Mighty and Exalted: "Today I have completed for you your religion" (al-yawma akmaltu lakum dīnakum). And it is said: the meaning of "Today I have completed for you your religion" is that He has made obligatory what you need in your religion, and that is a sound and acceptable interpretation; as for God's religion being, at some point in time, incomplete — that cannot be. And they say, "al-yawma yawmuka" ("today is your day"), meaning to express disapproval and to magnify the affair. And in the ḥadīth of ʿUmar, God be pleased with him: "the sāʾiba (freed camel) and the ṣadaqa (alms) are for their day (li-yawmihimā)," meaning the Day of Resurrection — that is, the reward of that Day is meant by them. And in the ḥadīth of ʿAbd al-Malik: he said to al-Ḥajjāj, "Set out for Iraq, sleep sparingly, for the day is long (ṭawīl al-yawm)"; it is said [entry continues]