Well with my somewhat expanded knowledge of Swedish, you should be able to use i which basically means "for the period". I think under works best here though. I don't think you can use på here but hopefully a native will correct me if I am wrong.
"På" is used for an unset amount of time, so having it for a set amount of time is strange.
Also, "i" can be used, but it's very strange. It's be like saying "Inside of the period of the month" Correct, yes. Strangely worded? Definitely.
I have a question about capitalization in Swedish. In English, you capitalize languages, months, and days of the week, but in Swedish you don't. Why is this and are there any other places where this happens?