There's a few possibilities:
- March 2021
- June 2021 (WWDC)
- November 2021
March or June seem the most likely. With all of the videos showing the MacBook Air beating the 16", I'm sure Apple wants to get their larger MacBook Pro back on the top end where it belongs. The sooner they do this the better and March would allow them to bring the 14", 16" and new iPad Pro out in one sleek "Pro" event. Launching in June would also be a good plan since it gives software developers even more time to get Apple silicon native apps out there. The 16" is marketed at a more pro user base so having apps like Maya and Photoshop ready to go on day 1 would be a big selling point.
IMO November is a little less likely. According to rumors, Apple has several new major Macs coming in 2021. Some reports even say we may see a new Mac Pro. Iff that is the case, I expect Apple will save the new iMac and Mac Pro for their November event and use a different event to introduce the new laptops.