Is the word when a relative pronoun?

It is a relative adverb when it is the object of the verb of the main clause, e.g. I don't know when they will arrive. When it connects two full clauses, it is a conjunction, e.g. He opened the door when she knocked.

Some might not like the idea of an adverb being an object. In the above example: "... know when...," "know" is transitive, and the only possible oblect is "when." The only plausible solution for those who can't accept an adverb as an object is to decide that, in this instance, it is a pronoun.

On the other hand: "In the days when I was young..." "When" replaces the noun "days." It acts as a pronoun. I have changed my mind.

Yes, "when" can be a relative pronoun.