I think "tomorrow is another day" is a nice workaday version of the old classic "this too shall pass." If you had a good day, that's swell and all, but you still have to plan for tomorrow. And if you had a crappy day, that sucks, but tomorrow you get to take another pass at it.
"It is what it is" is harder to defend. But the kernel of it which I appreciate is: you have to accept the basic facts of the situation you're in. You can try all the different dressings you want, but if your lettuce is rotting you'll never have a good salad.
I also appreciate that prhase in the way Dana mentioned, i.e. with the unstated preface: "I would prefer this were not the case, but it is what it is."