I think that the best way to interpret The Sun card is to consider the way the sun itself operates, the energy it offers, the way it lights up the earth and the warmth it gives and takes as it rises and falls. To consider that day gives way to night which again turns into day is to understand the essence of the card, which is a symbol of hope and beauty but isn't without its dark side. Many guidebooks on Tarot will herald The Sun as a wholly positive card and state that when it appears in a reading it means everything is coming up roses and positivity abounds in the querent's life. I find this kind of interpretation too easy. Taking a view that any card in the deck is wholly positive or negative stunts our ability to use the cards to maximum effect.
So, what is The Sun's message? Namely, it's about growth and development. It encompasses the feeling we have when we're going up in the world. Think plants rising up under the sun's nurturing presence. Think crops prospering and people sitting outside together having a good time, soaking up the rays. Plans come together under this influence and you can afford to act decisively and work diligently towards goals because you're capable of being the best you can be. There's also a suggestion of support and approval from others, which will strengthen your resolve and self-belief.
Which lessons from the past can you draw on in order to make the best of your future? You deserve to take this positive energy and turn it into concrete results, but that means letting go of what's holding you back and The Sun has its place in showing you how that can be done, by letting the darkness of night go and allowing the light to pour through your open windows. Allow the fresh, new feeling of potential. Don't try to stifle it by telling yourself you're not ready for it or worthy of it.