اليَوْمِ

A word in the Arabic language will either be 1) a verb or 2) a noun or 3) a particle – اليَوْمِ is a noun

A noun may either be 1) Definite or indefinite 2) Masculine or feminine 3) Singular, dual or plural 4) Nominative, accusative or genitive

اليَوْمُ is the natural state of this definite noun and it changes to اليَوْمِ due to: 1) a preposition that precedes it or 2) a possessed noun that precedes it or 3) it’s a noun of dependent declension a) نَعْتٌ / صِفَطٌ adjectives b) التَّوْكِيْدُ emphatic c) المَعْطُوْفٌ conjunction d) البَدَلُ synonym