Ukrainian likes to loose with word order, I'm sure for almost any Ukrainian sentence there's a myriad of ways to order it! It's one of the reasons why Ukrainian is such a beautiful language.
All Slavic languages have a loose word order, one of the few advantages of having the grammar cases :-)
No, in this case it's clear :)
"Вівторок був учора" -> Tuesday was yesterday (Tuesday is the subject)
"Вчора був вівторок" -> Yesterday [it] was Tuesday (Yesterday or "it" is the subject)
Given a very specific intonation and given a specific context, a pre-existing conversation, the second one could mean the first one, it would emphasize that Tuesday is still the subject, just shuffled around.
In case of п'ятниця it's "Вчора була п'ятниця", so the verb corresponds to the day of the week, not "yesterday", so... Yesterday is not a subject.... But the two sentences still do sound different... But... * leaves tired and confused *
I'm assuming the verb is modified by the day of the week, and not the word for yesterday based on other examples. Is that correct?