observance vs. observation

observance (noun):
the practice of following a custom, rule, law, etc.
an act that is part of a ceremony or ritual
a customary practice, rite, or ceremony observances
a rule governing members of a religious order
an act or instance of following a custom, rule or law: observance of the speed limits
an act or instance of watching
observance  of this family tradition would make your grandmother very happy
observance of the smoking ban in public buildings
observation (noun):
a statement about something you have noticed : a comment or remark
the act of careful watching and listening : the activity of paying close attention to someone or something in order to get information
something you notice by watching and listening
an act or instance of observing a custom, rule, or law
an act of recognizing and noting a fact or occurrence often involving measurement with instruments 
a record or description so obtained
a judgment on or inference from what one has observed;broadly :  remarkstatement
the condition of one that is observed 
— ob·ser·va·tion·al  adjective
— ob·ser·va·tion·al·ly adverb
I'm not criticizing that kind of clothing. I'm just making an observation about the style.
Her constant observations about the weather bored me.
These facts based on close observation of the birds in the wild.
Observations made using the telescope have led to new theories.
Some interesting observations came from the study.
He recorded his observations in a notebook.