By default, the M-x calendar interface differentiates weekdays from weekends by applying a gray color to the former and a faint red to the latter. The idea for this approach is that the weekend should serve as a subtle warning that no work is supposed to be done on that day, per the design of traditional calendars.
Users who prefer all days to look the same can configure the variable
calendar-weekend-days to either use gray of weekdays or the faint red of
weekends uniformly.
;; All are treated like weekdays (gray color) (setq calendar-weekend-days nil) ;; All are treated like weekends (red-faint color) (setq calendar-weekend-days (number-sequence 0 6)) ;; The default marks the Saturday and Sunday as the weekend (setq calendar-weekend-days '(0 6))
For changes to take effect, the Calendar buffer needs to be generated anew.