observance

These two words share the meaning "close attention," though observation is much more common: our observation of the habits of the condor;the child's observance of the waving flags. If you refer to "compliance," "ritual," "celebration of religious rites," or "a rule of a religious order," the only word to use is observance, as in observance [not observation] of the law;church observances [not observations] such as baptism and Communion;followed the observances [not observations] of the Jesuit order. If you refer to "a remark or comment" or "a record of something seen or studied," observation is the correct choice: made a few casual observations [not observances] about the foul weather;astronomical observations [not observances] in one volume.