You have raised a number of issues and potential issues, but also still not provided any information about the reason for the initial "meeting"" request. Was any further explanation provided for why you needed to meet with HR and why they were insisting it be on a day off? Does the company have any specific policy stating that all meetings with HR will take place on employees' days off? (Even if they did, which I doubt, this would not make it okay.)

I don't know if this is a concern for you, but if you feel you were coerced into quitting then you still may be eligible to collect unemployment benefits, even if you technically left "voluntarily". What you are describing sounds like it could fall into the category where you would not be disqualified. You might want to start there and then look further into any reasons or pretext your company may have had for "forcing" you to quit. You need to ask yourself what your damages are and what it is you'd be looking for.

The next question regarding the "termination" would be whether you feel that you were the victim of any kind of discrimination and/or whether the company violated any public policy in getting rid of you. The most common example of that is "whistelblowers", companies wrongfully terminating soemone who reports illegal activities or refuses to take part in them can not be fired on those bases. There are somplicated issues involved & it may be worth at least consulting with someone.

Good luck.