It is the day when Siddhartha, a human being, achieved His 'highest potential'. It is him who was clear what he wanted and went after it with 100% surety, courage, discipline and commitment. NOT Buddha!
Buddha is not a human it is in fact our buddha nature. The nature/consciousness that is always there to help us achieve our highest potential i.e. itself.
When we see it this way we realise that Buddha has been within 'each one of us' since the day we were born. It's the voice within us, which when we listen with attention sometimes, nudges us to move towards our own highest potential!
(Of course we are free to define our own true potential and then go after it like he did.)
Also The Bodhi Tree he sat under was not just a tree...it represents the shadow and support of all the buddhas and potential buddhas... people he ever came in contact with, his family, ancestors, the jungle, the trees, the birds, the water etc etc... and all the enlightened beings who ever existed. So we need to understand this and know the interconnectedness of all things. Nothing is possible alone.
So today is the day to celebrate the fact that ANYONE CAN achieve his/her highest and true potential.
EACH ONE of us CAN!
Gautama proved it to us that each of us has the capacity to realise our highest potential.
I sincerely wish you all a progressive life ahead and that you can also celebrate your Own Day! soon...not necessary a Purnima day! :)