I thought "jest" would always be followed by the instrumental case? Shouldn't this be Jutro jest sobotą?
Not always. For example, in "This is Y" sentence Y is in Nominative, and even in "X is Y" sentence Y is in Nominative if it's just an adjective.
In fact, you can say that Y is in Instrumental if both X and Y are noun phrases (or X is a personal pronoun and Y is a noun phrase).
So, as "jutro" is an adverb, that doesn't work. In fact, it would be like defining "tomorrow" by being "a Saturday". Which is not true. Tomorrow happens to be Saturday, but it's not like every tomorrow is a Saturday.
More info here: https://www.duolingo.com/comment/16373167
but could it still technically work tho? the same way as "zwierzę jest kotem" works, it just doesn't make sense semantically
"zwierzę" is at least a noun.
In a way, we could say that 'technically' it could work, but no, that's definitely unacceptable here.
it would still be accepted though as the program doesn't mark you wrong for accents ;)
Monday - (ten) poniedziałek - masculine
Tueasdy - (ten) wtorek - masculine
Wednesday - (ta) środa - feminine
Thursday - (ten) czwartek - masculine
Friday - (ten) piątek - masculine
Saturday - (ta) sobota - feminine
Sunday - (ta) niedziela - feminine
Do these sentences actually change based on the day you see it, or are they fixed? I thought they were fixed sentences, but I just realized it's never been wrong (at least with days of the week). Could just be a coincidence.
why is it sometimes jutro and other times jutra? does it adapt to the word thats it with?
Sometimes it can be treated as a noun, then it adapts to the case that is needed, like "od jutra" (since tomorrow) or "do jutra" (until tomorrow) take Genitive.
Mostly it's used as an adverb though, and then it does not change.