Tsugomori,' a kun-yomi (Japanese reading of character) for kaijitsu, once was 'tsukigomori' (the disappearance of the moon), which had originally meant 'kai' in the moon phase.
晦日にあてられる訓読みのうち、「つごもり」は「月隠り（つきごもり）」が転じたもので、本来は月相の「晦」の意味である。