MiniCalendar Weekdays Markers
The Weekday Headers $a and $b correspond to the settings in Current/Other Mini-Calendar. The a and b correspond to the LanguageTags a and b respectively. Note that $a can also be written as $$ if there is only one language.
Weekday Headers are evaluated in the order they appear, based on the Week Starts MiniCalendar setting. If you wish to only show some day, Sundays for example, you should place all 7 of the $$ symbols, but then make the first 6 invisible, or use the Day Headers options of the MiniCal property.