Very useful advice from Merlin in this week's episode of Back to Work on making sure that your calendar contains stuff that has to occur on that date/time. Why? Well, as he says:

nothing goes on the calendar unless it's something you have to do at a certain time on a certain day

and

we have this habit - when we procrastinate - [to] prioritise things we've been putting off by putting them on the calendar and say "I'll definitely write 50 pages of my novel" on Tuesday at 9am. But your brain knows that you don't really have to do that.

Guilty as charged. I moved ~10 "stuff I have to do" appointments from my calendar to OmniFocus last night. Next month's calendar is much cleaner and, more importantly, only shows stuff that must be done on that day and time.