which is the true order of a day, sunset to sunset or midnight to midnight and why?

Sunset to sunset, as it was ordained.

Midnight to midnight is man's invention of reckoning time... which has been erroneous ever since. Yet we all continue to follow it. Why is that?

There is no right or wrong way. The Bible describes the day as beginning at sunset. It lists evening first. It says in Genesis 1, "There was evening, then there was morning, the first day." And you can see if you read about the ordained holidays of the Bible that the day was reckoned to begin at sunset.

But for others, the day begins in the middle of the night! At midnight. I am unsure of the historical reasoning for that. For others, it begins at sunrise. That seems to make the most sense to me.

But there is really no right or wrong way. There is nothing inherent in any of these reckonings. A day is a human word, and it just depends on how you wish to define it.

Midnight to Midnight, 12am to 12am =24 hours, 24hrs= 1 day.

military time, after 12pm it's 13, 14, 15 hrs and so on. A day is completed when it reaches 24hrs.

midnight to midnight and because for the western calander the day ends at 11:59:59 pm and the next day starts at 12:00 am

There is God's calendar & God's schedule of appointed times.

And there are pagan calendars & pagan holidays.

We have the freewill to choose which/Who we will honor by considering it True.

i have no idea. sorry.