Working on the calendar.less in a proposed re-style in MDL-44328 it was pointed out to me that the changes I was making, that of converting calendar event background colours into LESS variable and also defining the 'weekend' colour, would be better in a moodle_variables.less (or variables_moodle.less) file:
 

@calendarEventCourse:   #ffd3bd;

@calendarEventGlobal:   #d6f8cd;

@calendarEventGroup:    #fee7ae;

@calendarEventUser:     #dce7ec;

@redDarker:             #a00;