Learn Russian: Это обыкновенный день.

You would add the word самый (the most) right before the adjective. Because день (day) is a masculine noun, you use the masculine form самый. The new sentence becomes: Это самый обыкновенный день.

I usually see обычный for 'usual' or 'ordinary'. How is обыкновенный different?

They are very close in meaning and often interchangeable. Обычный generally relates to habit or routine ('usual', 'typical'). Обыкновенный emphasizes that something is plain, average, or 'nothing special' — which fits the vibe of this sentence perfectly.

Why does обыкновенный have a masculine ending when день ends in a soft sign (ь)?

Nouns ending in a soft sign (ь) can be either masculine or feminine in Russian, so you have to memorize their gender. День is masculine. Therefore, any adjective describing it must take a masculine ending, like the -ый in обыкновенный.

Why do we use the neuter word Это instead of the masculine Этот to match the masculine noun день?

Here, Это means 'This is' and acts as a generic pointer to introduce the whole sentence. It stays neuter in this role. If you used Этот, it would attach directly to the noun, meaning 'This ordinary day...' — which is just a phrase, not a complete sentence.