I know I've been there before where I tell myself I'll start a new "diet" next Monday. And after a couple days I would be tempted, and not only would I make unhealthy choices, but I would also have excessive amounts of what I wanted. And instead of chalking it up to a bad decision for one meal, I would tell myself since I had just blown my "diet", I'll start another day.
Planning is an important key to getting through these tempting situations, but I also know sometimes things come up unexpectedly and you don't get to really plan. So, plan as much as possible, and even if you give into your temptations, don't worry about. Don't let that one instance control your decisions for the rest of the day, week, or your life for that matter.
And why do we wait for a specific day? It seems Mondays are a popular day to start a "diet" or fitness program. Does it really matter when we start making healthier choices? When you're ready to start making the changes you need to live a healthier life, there's no perfect day. I've mentioned this before, there will always be something coming up. Holiday dinners, family gatherings, hanging out with friends, etc. are things that you will be a part of, and it's important you enjoy these. So, don't put it off until "someday"; because, the sooner you start making better choices, the sooner you'll start seeing results!