As part of our volunteer-driven accessibility initiative in GNOME Calendar, and for the first time in the 10+ years of Calendar's existence, we finally completed and merged the first step needed to have a working calendar app for people who rely on keyboard navigation. This merge request in particular makes the event widgets focusable with navigation keys (arrow left/up/right/down) and activatable with space/enter. This will be available in GNOME 49.
Most of GNOME Calendar's layout and widgets consist of custom widgets and complex calculations, both independently and according to other factors (window size, height and width of each cell, number of events, positioning, etc.), so these widgets need to be minimal to have as little overhead as possible. This means that these widgets also need to have the necessary accessibility features reimplemented or even rethought, including and starting with the event widgets.
We also hope to get other parts of GNOME Calendar accessible before GNOME 49, but I can't promise anything at the moment. We did start working with making the month view accessible: https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/564