Happy Disability Pride Month everybody :)
During the past few weeks, there's been an overwhelming amount of progress with accessibility on GNOME Calendar:
• Event widgets/popovers will convey to screen readers that they are toggle buttons. They will also convey of their states (whether they're pressed or not) and that they have a popover.
• Calendar rows will convey to screen readers that they are check boxes, along with their states (whether they're checked or not). Additionally, they will no longer require a second press of a tab to get to the next row; one tab will be sufficient.
• Month and year spin buttons are now capable of being interacted with using arrow up/down buttons. They will also convey to screen readers that they are spin buttons, along with their properties (current, minimum, and maximum values). The month spin button will also wrap, where going back a month from January will jump to December, and going to the next month from December will jump to January.
• Events in the agenda view will convey to screen readers of their respective titles and descriptions.
Accessibility on Calendar has progressed to the point where I believe it's safe to say that, as of GNOME 49, Calendar will be usable exclusively with a keyboard, without significant usability friction!
There's still a lot of work to be done in regards to screen readers, for example conveying time appropriately and event descriptions. But really, just 6 months ago, we went from having absolutely no idea where to even begin with accessibility in Calendar — which has been an ongoing issue for literally a decade — to having something workable exclusively with a keyboard and screen reader! :3
Huge thanks to @nekohayo for coordinating the accessibility initiative, especially with keeping the accessibility meta issue updated; Georges Stavracas for single-handedly maintaining GNOME Calendar and reviewing all my merge requests; and @tyrylu for sharing feedback in regards to usability.
All my work so far has been unpaid and voluntary; hundreds of hours were put into developing and testing all the accessibility-related merge requests. I would really appreciate if you could spare a little bit of money to support my work, thank you 
• https://ko-fi.com/theevilskeleton
• https://github.com/sponsors/TheEvilSkeleton