"On Friday the letter comes."
Translation:På fredag kommer brevet.
When there is an adverb at the beginning of the sentence, the verb comes before the subject. In this sentence, "på fredag" is an adverb prepositional phrase, "kommer" is the verb, and "brevet" is the subject. Because "på fredag" is at the beginning of the sentence, "kommer" goes before "brevet." Another example: Nu äter jag (Now I am eating). "Nu" is an adverb at the beginning of the sentence, so "äter" must come before "jag" as opposed to the usual other way around.
Week days take på, just as they have on in English. Seasons are also på, på sommaren = 'in the summer', but months are in: i februari = 'in February'.
om with time usually means 'after a certain amount of time', like Brevet kommer om en vecka 'The letter will come in a week'.