Regardless of what happened today, tomorrow is on its way - OMGoodness I didn't realize that was going to rhyme. Regardless it's true. Tomorrow can be better, tomorrow things can change. Tomorrow whatever it is that you've been putting off can start. It doesn't mean that today doesn't matter, it just means that IF you want to change something, if it's finally time, tomorrow is as good a time as any to do it.
The sun sets, the stars come out and then the sun pops up again in the morning. It's the most simple, extremely obvious way for us to be reminded; tomorrow you get to start all over, you get to try again.