It really exists, it is not tied in to anything or anyone other than the position of Earth
in its orbit around the Sun, and it is the original reason so many holidays are focused
around this time of year anyway.

Tying a major holiday to any one person will only cause more problems and future
generations will end up "worshipping" them, too. Newton may have been a smart
guy, but he was still a mortal and quite frankly I never liked how he treated others,
especially those who "dared" to come up with similar ideas at the same time as him.
Just ask Leibniz, Gray, or Flamsteed.

Plus while he was running the English Mint, he played a major role in sentencing
several counterfeiters to death. Bit extreme, doncha think.

Just celebrate the Universe or Humanity in general.