Learn Danish: Det var en speciel dag for os. - It was a special day for us.

Both mean "special," and in this context, they are largely interchangeable. However, særlig often leans towards "particular" or "distinctive," while speciel can mean "out of the ordinary" or "unique." When talking about a memorable personal event, en speciel dag is very natural.

Why does the sentence start with det when dag is an en-word? Shouldn't it be den?

Det functions as a dummy pronoun here, just like the English "it" in "It was a special day." When setting up a sentence with the verb at være (to be) to introduce or define a noun, Danish always uses det, regardless of the gender of the noun that follows later in the sentence.

Why is the adjective speciel and not specielt?

The adjective must agree with the noun it modifies. Since dag is a common-gender noun (an en-word), the adjective takes the base form speciel. If the noun were neuter (an et-word), like et øjeblik (a moment), it would be et specielt øjeblik.

Why do we use the preposition for instead of til here?

When expressing that something holds a certain emotional significance, experience, or value to someone, Danish usually uses for (like det er vigtigt for mig meaning "it is important to me"). Til is typically used for physical direction, giving a physical object to someone, or indicating purpose.