Weekdays will be quieter but if the weekend is your only option it would be best to turn up as early as you can. It's fairly safe to say Saturdays and Sundays will be the busiest, but it's hard to predict how busy car parks are going to be on any particular day.
Last Monday I arrived at the small car park for Glen Callater at 9 30 AM, en route to Lochnagar and it was only half full. On Friday the North Face car park for Ben Nevis was close to full at 7 15 AM