This is the 90th most frequent Finnish word.
mennessä
By (a certain time); by the time.
Here, 'mennessä' is used to specify the deadline or the latest time by which an action should be completed.
Saavu paikalle ennen viittä, viimeistään kello viiteen mennessä.
Arrive at the location before five, at the latest by five o'clock.
In this context, 'mennessä' indicates the time by which the action of collecting trash must be finished.
Kaikki roskat tuli kerätä pois iltaan mennessä.
All the trash had to be collected by evening.
In this situation, 'mennessä' expresses the time limit for submitting the answer.