Why is 'de kommer i torsdag' incorrect?
på is just the preposition to go with days of the week, just like it's on in English and not In.
With months, it's i in Swedish and in in English.
Why is it kommer when they aren't coming right now? I thought it would be komma, as they're going to come on...
》In Swedish (& Finnish, etc.) future actions can often be implied with present tense.
I think is på torsdagen.
That would be if you’re contrasting it with other days or like making a schedule. Like På onsdagen handlar vi och på torsdagen kommer de. (On the Wednesday we go shopping and on the Thursday they come*. Otherwise you don’t use the definite.