For those who are not familiar with this, the Ceremony of the Keys is the traditional locking up of the Tower of London, and it has taken place every single night for over 700 years. There are limited tickets available so you must book ahead at the first opportunity at this website: https://www.hrp.org.uk/tower-of-london/whats-on/ceremony-of-the-keys/#gs.4ewlu5
For the past few years, you have been able to book online beginning on the first business day of the previous month. So for example, tickets for December opened up on the first working day in November. The booking started around 12 noon GMT, which of course meant that most people from the states had to get up at the crack of dawn to have a shot at it. And sometimes it was delayed.
This recently changed. Now booking starts promptly at 16:00 (4 pm) on the first business day of the previous month. So I was able to get online today at 10:00am to get a March date, which was nice. The process is simple. If you go online to the site before it begins, you just sit there and wait online. There is a countdown that will let you know how much time is left. You can have them send an email if you want that will let you know when it's time, but I chose to just wait.
At exactly 10:00 Central time, it opened up and I was assigned a random place in the queue (no. 924). It went quickly, however, and I got to my turn within 40+ minutes. There was still a decent amount of tickets left and I got my first choice. I paid for it (£5) and downloaded the ticket. There are still tickets left if anyone is traveling in March. Keep in mind that business days not only mean Monday through Friday, but also any bank holidays, etc.
A couple of notes: If you plan on attending this, make sure you read the rules at the link in the first paragraph. Also, if you're interested, here is a screenshot of what you will see prior to the booking opening up. https://photos.app.goo.gl/QRns5ys1nnZbhBGg8