Learn Danish: Jeg husker den dag hvor vi mødtes. - I remember the day when we met.

Usually, "the day" is dagen. However, when you point out a specific day that is immediately followed by a descriptive clause (like "when we met"), Danish uses the separate article den. It feels a bit like saying "that day".

Doesn't hvor normally mean "where"? Why does it mean "when" here?

Yes, hvor usually asks about location. But in Danish, when you refer back to a specific time word (like den dag or det år), you must use hvor to mean "when" or "in which".

I learned that da means "when" for past events. Could I use da here instead of hvor?

Not in this exact sentence, because hvor is acting as a connecting word specifically for the noun den dag. However, if you remove "the day" from the sentence, you can absolutely use da: Jeg husker, da vi mødtes (I remember when we met).

Why does the verb mødtes have an "-s" at the end?

The "-s" makes it a reciprocal verb, meaning the action happens between two or more people. Vi mødte would mean "we met (someone else)", but vi mødtes means "we met each other". Mødtes is the past tense of mødes.