Take a deep breath right now, and close your mind. Stop thinking. Stop thinking about the bad that happened yesterday, or the good that you're looking forward to. And feel yourself in this particular moment, not dependent on any other, just this moment, just you. Feel at home.
Just how many times do we define our 'today' by the after effects of 'yesterday' or the nervousness of 'tomorrow'. It is reflexive, we (atleast I) are constantly judging whether we are happy or not and the judgement is based on the premises of what was or will be. The continuity is everlasting, if you can't be happy now, you will carry this gloom into the next moment and into your own future.
I am no philosopher, but I can say this, if you want to be happy tomorrow, just dwell in the riches of the present and be happy right now.