Deep Winter day

"Deep Winter day" I used it earlier this evening simply because I liked the way it sounded. It fit my need to define something as 'cold', as in shunning or rejecting someone.

Found more uses of the phrase than I thought I would through Google. But no real explanation of what it might mean.

Any thoughts...?

"In the deep of winter" is a fairly common phrase which I have always taken to mean, "a long time since the last significant period of above-freezing temperatures." Does that mean also with no signs of an imminent thaw? I don't know about that. Thaws come unpredictably, except for the "January thaw." And January ought to be the best time to be "in the deep of winter." I also associate "the deep of winter" with snow. But then, I grew up where there was lots of snow in the winter.
SS