Translation:Stores are open all Sundays in December.
because "are opened" does not make sense in English. It would need to be "are open" or "were open" or "opened" to be correct English. Here, "are open" is correct because "aperto" is being used as an adjective, not a verb. "Aperto" as a past participle of the verb aprire conjugates with avere not essere in the perfect tense, this is the clue that this sentence is not in the past tense.
Yes shouldn't this be "opened" if some part of the Italian sentence is past tense? At the very least that answer should be accepted. Otherwise this is just present tense and the Italian is wrong! Also the lack of article is making me angry because anytime i translate a sentence without one DL marks it wrong no matter how awkward. Have some consistency!
This sentence should not be in the passato prossimo lesson. This sentence is not in passato prossimo. It is as someone pointed out present passive. Aperti is being used as an adjective not a past participle. For this to be passato prossimo we would need to change it to "I negozi sono stati aperti..." The store had been open...