Μου αρέσει Δεκέμβριος - is not correct? Why?
In Greek the definite article is used with almost all nouns. So, here you needed "Μου αρέσει ο Δεκέμβριος". I've added the whole phrase to the drop down hints to avoid other problems.
It’s possible the phrase may appear in different exercises, because your hint was not in the drop-down menu.
The reason I missed it is because I confused it with the days of the week where Aύριο είναι Πέμπτη lacks the indefinite article.
is it understood then when saying "Μου αρέσει ο Δεκέμβριος" that I like December in general? Like the idea with the English sentence.
Yes, that's right
Ο Δεκέμβριος μου αρέσει is not okay?
Why some exercises require definite article with names of the months and some don't? Are there months used without definite articles? Thanks