Learn Swedish: Jag minns sommaren då vi dansade hela natten. - I remember the summer when we danced all night.

In Swedish, when you use a word for "when" to refer back to a specific time noun that was just mentioned, like sommaren (the summer), you use the relative adverb då. You generally use när for questions or general time clauses like "when it rains". Because we are specifying exactly which summer we mean, då is the right choice to link the clauses.

Why is the word for night in its definite form (natten) after hela?

When you use the word hela to mean "all" or "the whole", the noun that follows it must be in the definite form. That is why we say hela natten instead of just hela natt, which literally translates to "the whole the-night".

Minns ends in an -s. Is this a passive verb?

No, minnas (to remember) belongs to a special group called deponent verbs. These verbs always end in an -s but have an active meaning rather than a passive one. So jag minns simply means "I remember".