Symbolism, Imagery, Allegory
Remember the page with the snow?
Ha. We're joking. We know, as do you, that just about every page of The Snowy Day is blanketed with snow. That's apt, seeing as how this is a book about a snowy day, but you know what else just about every page in this book contains? Expressions of childlike wonderment. In fact, the only pages on which Peter is not marveling at something new or having a new experience are pages without snow (32-34).
The rest of the time he's either captivated by its possibilities (10, 12, 15), experimenting with its properties (16-20), or playing and reveling in it (24-28, 33). So it's pretty clear that the snow is a symbol for the magic and wonderment of youth. And that makes Peter's dream, in which all the snow is melted by the sun, all the more visceral. After all, who wouldn't have a little anxiety over the possibility of losing so much magic and fun (a.k.a. snow) so quickly?