Translation:I am busy on Monday.
Original sentence 月曜日は忙しいです
Since it's not specific which Monday it is, and we only have 1 sentence to work with, the assumption is every monday.
Changing to が means
[ 私は ] 月曜日 が 忙しいです
here, when が is used, means "I am busy on Mondays", as watashi is usually omitted, and in English, "every" is similarly omitted by context
Now, to be specific, if you want to say that your workplace is busy, or will be busy, "next" or "every" Monday;
And of course, if you want to say that "you" are busy, "next" or "every" Monday;
Why would you say that “y is a characteristic of x”? In “僕はパーティーが好きだ”, for example, you just mean you like parties, parties are not a characteristic of you.
I also think that “が” is wrong here, unless you're trying to say ”Monday(s) is/are busy/hectic”. I think a sentence not using “は” would have to simply be “月曜日ひそばしいです”, as time is generally indicated without any particle. Of course I may be wrong.
I disagree. Monday could be the implied subject, depending on context.
That's the whole point of the subject being "implied". The sentence could work either way. "As for Mondays, (they) are busy." or "As for Monday, (it) will be busy."
That works just as well as the alternative translation of "As for Mondays, (I) will be busy."