How is четверг supposed to be pronounced? Here, the stress is on the second syllable (четверг), but I think I have heard the stress on the first syllable (четверг).
There's no real explanation. In the sentences like this we use prepositional with months, locative with years (in Russian it's a variation of the prepositional rather than a case on it's own right), and accusative with days of the week. That's just how it is.
The days of the week are also based on numbers in Portuguese, but Sunday is the reference point. Thursday, for example, is "quinta-feira". "Quinta", its short version, also means "fifth" (feminine), since it's the fifth day from Sunday. Because of that, learning days of the week in Russian can get very confusing, since you consider Monday the first day of the week! :)
I have the same question as another sentence. I would say, what were you doing on Thursday