Christianity can Christianize anything
Most seasonal/earth elements to holidays are pagan.
Everything wasn't pagan. That's the whole point. What I see are two errors. One is to make everything Christian as coming from Judaism. The other is to make everything Christian as coming from paganism.
If one wants to be as accurate as possible
then those who defend either errors
will pounce on those who wish to see the truth of the matter!
The truth of the matter is that sometimes Christians can develop or grow their own stuff for their own CHRISTIAN(belief or custom) reasons
Thus, everything doesn't have to come by way of either Judaism or paganism(gentile). It can also come naturally by way of deriving what already exists within it's own Christian bosom.
It can create new stuff, new feast days from it's own self.