As other have noted, calendars are tricky things with MS Access. I'm enclosing a database with a custom calendar to show you one approach.
The conditional formatting will not work with my example because I'm using list boxes instead of subforms for each date. You could theoretically have subforms for each date but it would be really cumbersome, you might, however be able to change the background color of each subform, or, alternately have another shape around the date that would accept conditional formatting (either with VBA or the toolbar help)
Anyway, it's just one method among many you could use.
CalendarExample.zip