Learn Hardcore Norwegian: Hver dag er ulik.

Because hver already functions as the determiner. You cannot combine hver with another article. Saying hver en dag or hver den dag would be ungrammatical.

Why is ulik not ulike in this sentence?

Norwegian adjectives inflect for number and gender when used predicatively:

• Indefinite singular masculine/feminine ⇒ no ending (dag is singular ⇒ ulik)
• Indefinite singular neuter ⇒ -t
• Plural or definite ⇒ -e

Since hver dag is singular and indefinite, ulik remains without ending. (If you said alle dager er ulike, you’d use ulike in the plural.)

Could I say Hver dag er forskjellig instead of Hver dag er ulik?

Yes, forskjellig also means “different.” The nuance is:
• ulik often stresses that no two days are alike.
• forskjellig can simply mean “not the same; various.”

Both are correct, but hver dag er ulik feels a bit more literary or emphatic about uniqueness.

What’s the difference between hver dag and hverdagen?

• hver dag = each or every day (viewing days individually)
• hverdagen = the everyday, the routine day (definite noun referring to the concept of daily life)

When would you use hver eneste dag rather than hver dag?

hver eneste dag adds emphasis—“every single day” or “without exception.” It’s stronger and more emphatic than plain hver dag.

Can you say hver unik dag to mean “each unique day”?

Grammatically you can—hver unik dag is possible (adjective in attributive position must match gender/number: here masculine/feminine singular ⇒ no ending). But it’s uncommon because unik/ ulI k overlap in meaning. More idiomatic: hver dag er unik or simply hver dag er forskjellig.