There's just something that I love about the idea of "letting it" be a great day...
It occurred to me this morning that I used to tell everyone to "HAVE a great day"... then I went to telling everyone to "MAKE IT a great day", because I felt that was more empowering. But the subtle implication there is that you have to force it to be great, and go out of your way to make it so. Yes, there's responsibility for whether your day is great or not, but what if we took it a step farther?
Letting today be a great day implies that greatness is the natural state of things, if you would just allow it to be so. It implies that a great day exists to be had every single day, but the only thing that could ruin it is your desire to control outcomes and your way of perceiving it.
So, go with the flow today and let it be a great. =] It is the natural way of things, after all.