There's no specific date mentioned, suggesting Facebook will roll out the change gradually but globally.
The old version of Facebook on desktop uses smaller typefaces and has a narrower News Feed, whereas the ‘new Facebook’ widens the Feed, moves ads out your way and makes the fonts larger. It also cuts back and hides some menu items to try and simplify what is a busy app. You can also run it in dark mode.
But, as with every UI update to a popular social media platform, some people will be miffed that their classic look FB is about to bid them adieu.
In the new pop up you can opt to send Facebook feedback on why you are switching back (briefly) to the classic version. Make sure you give Zuckerberg and co. a piece of your mind. And then get used to the new look and forget you were ever angry in the first place, probably. If you're really livid, you could always delete Facebook.