Learn Swedish: Under högtiden festas det hela natten. - During the holiday, there is partying all night.

In Swedish, intransitive verbs (verbs that don't take a direct object) can be used in the s-passive to describe a general activity without saying who is doing it. When you combine the formal subject det with the passive festas, it literally means "it is partied," but translates naturally to "there is partying" or "people party."

Why is the formal subject det placed after the verb festas here?

Because of the V2 rule! The sentence starts with the time expression Under högtiden (During the holiday). This takes up the first position, so the verb festas must come second. The subject det is pushed to the third position.

Why does hela require the definite form natten here?

The adjective hel (whole/entire) requires the noun to be in its definite form when you are talking about the entirety of a specific thing. Therefore, "the whole night" or "all night" is translated as hela natten, not just hela natt.

Can I use högtiden to talk about my summer holiday in Spain?

No, en högtid specifically refers to a festival, feast day, or cultural or religious celebration (like Midsummer or Christmas). If you want to talk about your personal time off from work to travel, you should use semestern.