This is the 11788th most frequent Russian word.
закончатся
"Закончатся" means "will end" or "will run out."
Here, 'закончатся' is used as a future tense reflexive verb to indicate that something will end or run out on its own. Specific to describing the depletion of a finite resource like groceries.
У нас скоро закончатся продукты.
We will run out of groceries soon.
In this sentence, 'закончатся' describes the conclusion or ending of lessons. It indicates the completion of an activity.
Когда уроки закончатся, мы пойдем гулять.
When the lessons are over, we will go for a walk.
Here, 'закончились' (past tense of 'закончатся') is employed to express that the vacation days have come to their natural end. The word is in past tense and reflects on a completed action.