Is your feature request related to a problem? Please describe.
Currently the days of the week aren't announced by a screen reader when navigating the calendar. The <table> structure is correct, so presumably this is due to the use of grid + gridcell roles. Whatever the cause, the result is that screen reader users don't know what day of the week their currently-focused date is.

Describe the solution you'd like
Simplest solution would be to append or prepend the day of the week to the hidden text in each button, i.e.:

<span class="duet-date__vhidden">Thursday, 2020-09-24</span>

This could be done with Date.toLocaleDateString() to support internationalization.

Describe alternatives you've considered
The only alternative I can think of would be to drop the use of grid and just use a plain table, but this doesn't seem like a good solution—it would mean sighted keyboard users would be forced to TAB through the calendar instead of using arrow keys.

Additional context
Tested with JAWS, NVDA and MacOS Voiceover.