This is the 288th most frequent Russian word.
понедельник
Monday.
Here, 'понедельник' is the subject of the sentence, indicating the specific day.
Понедельник — первый день недели.
Monday is the first day of the week.
In this sentence, 'понедельник' is preceded by a preposition 'в', which contextually modifies it to express 'on Monday'.
В понедельник я пойду в театр.
On Monday, I will go to the theater.
Here, 'понедельники' is in its plural form to generalize feelings towards all occurrences of Monday as a weekday.