You will not be less busy in the future than you are right now.
This rule addresses the illusion that there will be a less busy time than right now, located somewhere in the future. I know a guy who plans on writing his book when he retires from teaching! Many people tell themselves they will start working on something once the present business is over, but it never is. During my busiest week I often get an article to review for a journal. I still do it that week, because it is easier to get a lot done when you are already very busy. When it's harder to get things done, paradoxically, is when you don't have as much to do.
By postponing tasks to supposedly less busy times in the future, you are essentially making those hypothetically less busy times more busy, cluttering them up in advance. Do you really want to do that?