Give you an example from the diet side. Let's say that you have decided to lose 10 pounds over the next few months. You've picked a diet that works for you and joined the gym. Your plan is to diet daily and work out at least twice a week.
For the first few weeks things go really well and life is good. Your able to hit your diet goals and make it to the gym right on schedule - then real life hits. Work tosses you a curve, and you end up working late to get the project done on time. Of course, its also one of your Gym nights, and its late. You want to go home and forget about work.
Do you go to the Gym?
It's easy to rationalize not going. Your tired, you don't want to, the day was hard and you can go some other day. Although accurate, it's a bad idea to not go, especially when you are starting a new routine. If you skip one day, diet or exercise, why not two? Three?
As you start working out, dieting, or any new goal it is very easy to stop. What we have to realize here is that the most important thing isn't that you get to the Gym or stay on the Diet it's that you establish a new habit. It's that habit that will serve you better than any diet-du-jour or workout-of-the-month. If you do it consistently, whatever it is, you will meet your goals.
That's why the best time to diet, work-out, mow the lawn, etc. is when you don't want to because in doing it when you don't want to you will push yourself to change the habit and do it anyway. Once the habit is ingrained, you'll be amazed at what you can accomplish.