I think you can even interpret "always Winter and never Christmas" in a seasonal way, because Christmas is historically tied to the Solstice

Oh yeah, oh brilliant! See, I tend to associate seasonality in epic fantasy with Michelle West, and in her books the winter solstice is Scarran, the dark conjunction, and it is a Very Bad Time in some specific circumstances and Winter magic is Very Not Good. I'm actually fairly certain that West is writing out of a Narnia influence to some extent.

It's kind of ironic that Jadis spent who knows how long in stasis in Charn, but that preserved her far better than being immortal.