For UK owners/riders, that's the date the new SORN (Statutory Off Road Notification) rules kick in.
After the 16th, you won't have to tell the government each year that your bike is off the road (not that anyone here would dream of leaving a motorcycle unloved, unused, and unabused in the garage for 12 months or more).
After the 16th, it's SORNED until you re-tax it or sell it. If you sell it, the new owner will need to re-declare SORN unless he or she taxes it.
Also, insurance certificates are history after that date. You'll still need insurance, and you'll want a certificate in the post as usual, but you won't need to show it again. It can go in the drawer.
Everything will be done electronically. So whether you tax online, or at a post office, your application will be automatically checked and approved/rejected.
I don't know if this has been mentioned here on this site recently, but it's worth a reminder perhaps.
And you all know, of course, that paper tax discs are also for the chop.
Horror, don't spread this around, but I've got a Bonnie in the garage that I rebuilt top-to-bottom many years ago, then rode for a year or two, then cannibalised some bits from it (for another Bonnie) and still haven't got it back where it belongs.