Difference between “на”, “для” и “за”.
These Russian prepositions have many different meanings, but you probably meant the temporal meaning here.
На is used in the situations when the action started and it still lasts. It is used with the Accusative case.
Мы приехали сюда на неделю. We came here for a week (We came here and we will stay here for a week).
Для does not have any temporal meaning, it means "for somebody" or "for something". It is used with the Genitive case.
Для кого этот торт? For whom is this cake?
За means "to finish an action within some time limits". It is used with the Accusative case.
Мы закончили работу за час. We finished the work in an hour.