When a calendar has disabled dates and a user navigates by keyboard, these dates get skipped. This is logical behaviour if we would see the calendar as just a set of buttons. Usually interactive elements that are disabled can't be navigated to.

In a calendar however, what we are mainly navigating is not the buttons, but the grid of dates. The buttons just happen to be in those cells. There is value in the coherence and structure of the grid, and being able to browse these disabled dates.

For example, a screen reader user goes through a calendar. They would like to select a Monday, but for several weeks, all Mondays are unavailable. In the current pattern, they would have to use right or left arrow and go through all weekdays before they find a Monday. If the disabled dates were navigable, they would be able to use the down and up arrow to go through just the Mondays.

I would suggest changing the keyboard navigation to include disabled buttons. For added convenience, I would suggest prepending something like "Unavailable, " to the accessible name of these dates.