User blog comment:RaythMercury/Translations/@comment-6916556-20150326103114

There are two voicings of Kanji on'yomi and kun'yomi. On'yomi is poetic or traditional voicing: like Getsuga Tenshō or Zangetsu. Tsuki and Getsu mean the same thing, moon. They share the same Kanji, just Getsu is On'yomi which is the traditional voicing based on Chinese and Tsuki is Kun'yomi (based on modern Japanese voicings).

Some character's Zanpakuto are On'yomi and others and Kun'yomi. You have to have a feel for the language to know where to read which.