Why does poniedziałek not take the instrumental case? Can't it be jutro jest poniedziałekem as well? Or... why is it in nominative case?
The way I understand this sentence is as follows: a literal translation would be "Tomorrow, it is Monday". In other words, jutro is just an adverb of time to give context, it’s not the subject of the sentence, then jest is used in the sense there is, which takes the nominative.
Exactly. I would even translate (if I were to do it really literally) the sentence into English as: ‘There is (going to be) Monday tomorrow’. Poniedziałek (‘Monday’) is the subject, and as such, is in Nominative. ;-)
By the way, note that the instrumental of poniedziałek is poniedziałkiem.
The e between the ł and the k disappears, it’s a very common phenomenon for a e between two consonants at the end of a word to disappear whenever an ending is added to the word. Moreover, when you add an ending starting with e to a word ending with k or g, you need to add an i in the middle, hence the ending is -iem.