Learn Finnish: Juhannus on merkkipäivä, jona mennään mökille. - Midsummer is an occasion on which people go to the cottage.

Jona is the essive case of the relative pronoun joka. We use the essive case with days, dates, and occasions to express "on which" or "during which". Here, it refers back to the noun merkkipäivä, meaning "an occasion on which".

Who is actually going to the cottage when the sentence uses mennään?

Nobody specific! Mennään is the passive present form of the verb mennä (to go). In Finnish, the passive is commonly used to express what "people" or "one" generally does. Here, it translates naturally as "people go".

Why do we use the allative case mökille instead of going into it (mökkiin)?

By convention, Finnish treats cottages and cabins as open properties or estates rather than just enclosed buildings. Therefore, you use the external locative cases for them: you go to the cottage (mökille), stay at it (mökillä), and leave from it (mökiltä).

What makes up the compound word merkkipäivä?

It combines merkki (mark, sign) and päivä (day). In Finnish, a merkkipäivä is a significant day, milestone, or special occasion, literally a day worth marking on the calendar!