As a general rule, letters are sent as standard post. This mean that you can expect that letters delivered to the Internal Mail Service on Monday or Tuesday will be delivered by PostNord on Friday, while letters delivered to the Internal Mail Service Wednesday, Thursday or Friday will be delivered on the following Wednesday.
If it is absolutely necessary that your letter shall be delivered the next day (Monday to Saturday), you can send it as a ‘quick letter’ (Quickbrev). Write the letter ‘Q’ at the top centre of the front of the envelope. Please be aware that this is expensive: a quick letter weighing up to 100 grams costs 27 kroner.