The Reason for the Season
In the United States, Christmas and cold weather go hand in hand like fruitcake and figgy pudding. The cold weather adds to the atmosphere of Ordinary People. Beth is emotionally frigid. Conrad is depressed and feels no warmth. The snow is just the frosting on this depressing cake.
Even though the Jarrett family is non-religious, they celebrate Christmas. To them, and to most of America, "Christmas means gifts" (12.2). The act of shopping allows Conrad to get outside of his own head and think of other people.
However, what Conrad really needs to be doing is figuring himself out, so all the shopping is an unnecessary distraction. Calvin lets Conrad take over Christmas because he has a big soft spot, "always want[ing] to buy some tree you feel sorry for" (13.23). Just like Charlie Brown.
Depending on your point of view, Cal encourages Conrad, or—as Beth might say—he enables him. Beth finds it "silly" (13.27) that Conrad bought such a big real tree, since there's an artificial tree in the basement, which she likes because it's fake. Like her. Ooh, better step away from the tree, because that was a big burn.