"Czwartek nie jest miesiącem."

Translation: Thursday is not a month.

[ha.di]

"Thursday is not a month",
what does this mean or tries to say?! In which context somebody may use this?!

[Jellei]

In a context when someone (most probably a child or a foreigner learning the language) confuses Thursday with some month ;)

[arminia11_web_de]

I do this all the time! I confuse Thursday and June in Polish.

[htecwik]

me too!

[JanSleumer1]

If it is only that....

[4d0dj3iF]

What about "Thursday is not the month"?

[Matthew62991]

Difference between miesiac and miesiacem?

[Jellei]

"miesiąc" is the basic, Nominative form, used mostly for the subject of the sentence or in sentences built as "Czwartek to nie miesiąc".

"miesiącem" is the Instrumental form, used mostly for noun phrases after the verb "to be", e.g. "Czwartek nie jest miesiącem".

"Czwartek to nie miesiąc" and "Czwartek nie jest miesiącem" are basically the same, but I'd recommend the second option.