EXCLUSIVE: Christmas in April? Why not — if it’s got Dick Van Dyke and Valerie Harper? Here’s a sneak peek at their new short film Merry Xmas, which is set to unwrap, er, unspool at the Tribeca Film Festival next month. Glenne Headly and Matthew Modine co-star in director Boman Modine’s comic look at how the holidays can be murder. Expect danger! romance! tragedy! and … junks. Joe D’Angerio and Matthew Modine produced the short. Have a look, but beware the “Jingle Bell Rock” in the background — it might get stuck in your head until you hear it again at Target in mid-October.