In "To an Athlete Dying Young," Housman uses shade and night as metaphors for death. Light often represents life and warmth. Night often represents the opposite. These metaphors help the speaker present the athlete's death in a gentler way, softening the emotional blow. That A.E. sure was a considerate guy.
Line 13: "Shady night," has "shut" the athlete's eyes. This sounds a lot nicer, a lot gentler, than a terrible grain silo accident has killed the young athlete, right? Night is when we sleep. That's nice. Shade is often a cool, comfortable escape from the hot Sun. That's nice. This metaphor is designed to make death no more threatening than a nap under a shady tree. Nice.
Line 22: The athlete, in his coffin, has been set "on the sill of shade." The "sill" is that bottom part of a doorframe. So, with "shade" still working metaphorically as death, the athlete has been placed at death's doorway—the gateway to the land of the dead. The mourner's set the coffin down at the mouth of the open grave or tomb. Spooky, right? "Sill of shade" sounds super-pleasant by comparison. Housman's metaphorical use of shade makes the athlete's transition from world of the living to world of the dead seem a bit less creepy. Thanks A.E.