Learn Finnish: Oletteko täällä tammikuussa vai helmikuussa?

The ending -ssa means 'in' (this is called the inessive case). To say 'in January' or 'in February', you take the name of the month (tammikuu, helmikuu) and add -ssa directly to the end to get tammikuussa and helmikuussa.

I thought the word for 'or' was tai. Why does this sentence use vai?

Finnish has two words for 'or'. You use vai in questions when you are asking the listener to choose between two options (are you here in January OR February?). You use tai in regular statements, or when both options could be true.

How is the word oletteko formed?

It comes from the verb olla (to be). Olette means 'you are' (used for multiple people, or formally for one person). Adding the question particle -ko to the end of the verb turns it into a question: 'are you?'.

Is there a pattern to the names of the months, like tammikuu and helmikuu?

Yes! In Finnish, all the months end with the word kuu, which means both 'moon' and 'month'. Tammikuu literally means 'oak month' and helmikuu means 'pearl month' (referring to frozen ice droplets on tree branches in winter).