"It's just another manic Monday
Wish it was Sunday"
A mood is the form a verb takes to express a condition. With the subjunctive mood, that condition expresses suggestions, wishes, uncertainty, or possibility. In the song, since it's Monday rather than Sunday, "were" is grammatically correct to express a wish rather than reality.
The subjunctive form applies usually to the verb "to be," as shown below.
If you remember nothing else about the subjunctive mood, keep in mind that it often expresses a wish rather than a reality. Let's go back to song lyrics: "If I were a rich man" is correct. "Wish it was Monday" is not.