What is it all about that people celebrate that day? It's about the birth of Christ and that's why whether it's on winter, summer or right date or not.
The spirit is the same and that is all about the birth of the Savior that came to save everyone from sins. Just like you and me, we've got our birthday celebrations and sometimes we miss the actual date and it's just an agreement for most of the parts of the world to stay on significant day to celebrate it.
To me, I don't really think it's necessary to celebrate the birth of Christ because Christ while on earth never celebrated his birth nor ask the Christians to celebrate his birth. The only thing Christ admonished Christians to do is the holy Communion as that's the only way to remember the significance of his death and resurrection. His resurrection is even more to be celebrated because for him to come in human form, died and conquer death by rising again is the greatest miracle that has ever happened after the before and after the death of Christ.