To put it simply, is it ending Dec 31, 2020 12:00AM or 11:59PM?
It would not really matter as the ''time bomb'' (Flash playback blocked by default) won't kick in until January 12th. I would assume the beginning of that day.
Support is officially over at this point, so this is kind of moot. As the person above pointed out, we wanted to give system administrators time to get back from the holidays and make any last-minute adjustments to their networks. Someone will inevitably be caught by surprise, and we didn't want to break things while people were on holiday. Flash Player will stop loading content from the open web in a few days.
For completeness, here's the original EOL announcement from 2017:
Here's the consumer FAQ:
Here's the enterprise FAQ: