This is the 346th most frequent Russian word.
неделе
The word "неделе" is the dative or prepositional case form of "неделя," which means "week" in English.
The word 'неделе' is used here in the dative case to indicate the time frame (next week) during which the event (going to the theater) will occur.
На следующей неделе я пойду в театр.
Next week I will go to the theater.
The word 'неделе' is in the prepositional case here, referring to the specific week in the past (last week) when the event (end of the trip) took place.
Моя поездка закончилась на прошлой неделе.
My trip ended last week.
In this sentence, 'неделе' is used in the prepositional case to indicate the current week as the context in which we have a lot of work.