London and York in Mid-May

Prior to a trip to London earlier this year I subscribed to the newsletter on this site https://www.ianvisits.co.uk/calendar/ and found it very useful. Found a couple of gems we visited.

I found Londonist very helpful when it comes to finding events there. It has some unique articles as well, but it also tells you week by week what's going on there. https://londonist.com

Time Out London also a good resource for finding events, and it's great for finding good restaurants as well: https://www.timeout.com/london/things-to-do

If you want some really interesting and somewhat obscure things to do in London, I highly recommend Atlas Obscura. It's a great place for finding really off-the-beaten-path events and sites. https://www.atlasobscura.com/things-to-do/london-england

And of course, the main London tourism website is an excellent resource for events. They're going to have events probably farther out than anyone else. https://www.visitlondon.com/things-to-do/whats-on/special-events/london-events-calendar

For York, the VisitYork official website is a good resource for events: https://visityork.org/events

Most, if not all of the above offer email subscriptions where you can get a weekly update of what's going on in London or York.