The poem opens with a crowded, glamorous scene: "Anyone who was everyone was there." It's a wry line, since it suggests a guest list of important people while saying nothing real about any of them. Then the speaker turns away from the crowd and addresses the moon instead, offering a compliment as if it were a fellow artist: "I love your new work / with shadows."
The joke is that the speaker is making party small talk, the kind of gushing praise people hand out at openings and gatherings, but aims it at something that can't answer. The moon is "wan-looking," pale and a little tired, which makes it seem as bored or out of place as the speaker might be. The speaker feels slightly detached, amused, and perhaps lonely in the middle of all those people. The mood is light and playful on the surface, with a quiet note of isolation underneath. Judging by the ending, the moon seems the only guest worth talking to.