Jung often referred to the dark side as "shadow", and he believed that not only individuals, but also whole nations, communities and groups had shadows that had to be encountered. He felt that the shadow has typically been demonized and "made evil," rather than viewed in a philosophical and more fair or equitable light.
The shadow is a "dangerous peculiarity and a valuable and congenial asset as well" and contains not only the undesirable elements of life, but also the "dark springs of instinct and intuition," which mere reasonableness could never call awake.
The Shadow is that part of us we fail to see or know, all that we would not want to be, and it is the sum of all those unpleasant parts of our personality that we prefer to keep well hidden, our baser, more primitive side. It is the inferior, dark, undifferentiated side, in conflict with the conscious Ego. The Shadow represents something inadequate and includes those qualities that clash with the established norms of social behaviour.
Some Jungians have written about the gold in the shadow, believing that what consciousness rejects is often the stuff of life that may give it its highest value. The unconscious introduces new dimensions and might have a very different moral view compared to the one conditioned by society.
Whilst exploring the Shadow, the goal is recognition rather than integration: accepting the negative and welcoming it as a pole of energy is less a true process of integration than a sort of coming to terms with the Shadow (just as the energy previously dispersed in the Shadow becomes exploitable by the Self), since eliminating it entirely appears an almost impossible enterprise. After all, for human beings to be complete they must, of necessity, have some deficiencies.
Bearing this in mind, in my opinion the key shot of the movie is when we see the name of the groundhog (Phil!), carved in wood, appearing right above Phil Connors' head. It is hard to believe this is a mere coincidence, and not to think that it might have sprung from the depths of the director's own concept of the Shadow.
For Phil, living the very same day over and over again will bring about Phil Connor's honouring and accepting ("seeing", as the Groundhog) his own shadow through a profound discipline. Moreover, this process will bring him love with the woman who has been with him all the time, and the "winter" of his heart and his soul is over.