What does "Monday takes a long time to come around" (to come around - to finish?)

"I've not voluntarily invited another human being across the threshold" (human being - means merely "a person", why did they use "human being", not simply "another person"? mean?

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In this context, @Kellyne is correct regarding the meaning of "come around" (it has other meanings that don't apply to this situation, and I'll add them to the end of this answer). I would only add that, when used to refer to the concept of "arrival", "come around" is also especially common when talking about visitors. If you want to let someone know that it's okay for them to come visit you without calling first, you might tell them they can "come around sometime". Or if you see a friend unexpectedly while you're on the street, and you want to meet up later at your house for a real visit, you might say, "Come around the house later and we'll catch up."
Similar to the concept of "arrival", you can also use "come around" to talk about repeated actions/events (for example, "I want to have traveled the world by the next time my birthday comes around."), including actions/events in the past that no longer happen (for example, "Now that my grandchildren are older, they don't come around anymore.").

For informational purposes, other meanings of "come around" that don't directly relate to this question are "to change one's position" (for example, "He didn't like her at first, but he came around eventually." means that the man didn't like the women when he first met her, but as time passed he changed his opinion of her, and now he likes her) and "to regain consciousness" (for example, "The hit knocked her out, but she came around in a couple of minutes." means the woman passed out after being struck, but she woke up shortly after").

I would disagree, though, with the second part of @Kellyne 's answer in this specific situation. Generally, yes, "human being" is a more detached, scientific way to say "person", but the use of "human being" over "person" is often done for emphasis, as well. For example, if a mother is frustrated with her adult child's inability to fold clothes, she might say, "How is it possible that a human being with thumbs can't fold a shirt!" Or, for example, if someone wants to emphasize that a place was completely devoid of people, they might say, "The town was completely deserted; there wasn't a single human being around for miles."
So to say "I've not voluntarily invited another human being across the threshold" is a much stronger sentiment than "I've not voluntarily invited another person across the threshold".