Hi! I can not explain why this construction is used, as very often in languages thes best answer would be - just because, we say like this. But the case is dative

По понительником = в каждый понительник

Ksenia thank you so much for your comprehensive humorous explanation !!!!!

That's just the different ways to say some information , to have an option of making language more beautiful with changing a little bit construction of sentences and harder for people who learn that language 😁 it's all about noun declensions and sometimes about the context For example we need to say that : I work on monday - я работаю в понедельник / the noun is in a singular form accusative case , more about the following Monday, may be the next you will not 1)Я работаю по понедельникам -I work on Mondays / plural form dative case = 2) я работаю каждый понедельник - I work every Monday // yeach, every Monday you need to pull your alarm in the wall and get up =) If someone wants to meet with you, and for example you don't have enought time on Monday, but the next day are free : У меня нет свободного времени в понедельник, но во вторник я свободен. / So your friend has no need information about all your Mondays, we speaking about the nearest days, but if you planning meeting every week use (2). Каждый понедельник я работаю , но по вторникам я свободен.