Though talk of the temperature peters out by the time the poem winds to a close, weather is the star of the show in the first part of the Magus's story. And it's pretty miserable weather indeed—frigid and slushy (remember, it's nearly Christmas, in the most literal sense of the term). Then, the weather in the poem suddenly takes a turn for the better, or at least for the less frozen. This thawing motif, on a whole, is a kind of conceit, or extended metaphor, for the coming transformation of the world (well, some of it) brought by Jesus.
Lines 1-5: The opening of the poem locks us into, literally, "the dead of winter. The world pre-Jesus is pretty barren and bitter in this poem.
Lines 8-10: In direct contrast to the actual weather, these lines depict a kind of luxurious (maybe even lazy) summer on the palace grounds, complete with girls and frozen desserts. It's what the Magi are missing as they trudge towards Bethlehem. But, as we find out later, this summer they dream of is a kind of false summer, a warmth that proves to be empty of any spiritual value.
Lines 21-22: Sudden seasonal change alert. There's also a nifty, sneaky allusion in here. In Renaissance painting, it was somewhat common for the coming of Christ to be depicted as a sudden movement from barrenness to abundance. (For instance, you'd have a painting of Jesus, and on one side of him it would be all wintry and gross, and on the other, it would be sunny and civilized.)