The distinction between these words has been explained and recommended in books on usage for more than a century. It is a simple one: observance is to be used in the sense "an act or instance of following a custom, rule, or law," as in "observance of the Sabbath" and "observance of the speed limit"; observation is to be used in the sense "an act or instance of watching," as in "observation of a lunar eclipse." This distinction is, in fact, usually followed in actual usage:
Her only hope lay in strict observance of court procedure —American Girl, March 1953
His observation of the readiness of churchgoers to accept Hitler —Current Biography, September 1984
But observance and observation are also sometimes used as synonyms. Many commentators acknowledge that such usage was once respectable (it occurs in the works of such writers as Shakespeare, Coleridge, and Macau-lay), but most of them now want it to be regarded as an error. A more reasonable view is to regard it simply as rare. The use of observation to mean "an act of following a custom or law" is especially uncommon:
The South has never been solemn in the observation of this sacred day —American Guide Series: North Carolina, 1939
The use of observance to mean "an act of watching" occurs somewhat more often, although it is certainly far less common than the use of observation in this sense:
... evidence of Inge's faithful observance of life — Current Biography, June 1953
... made paranoiac by his observance of my rage — Sally Kempton, Esquire, July 1970
Dictionaries treat such usage as standard.
You may very well want to keep observance and observation distinct in your own writing (you probably do so already without thinking about it), but there is certainly no rule that says you have to.(资料出处：韦伯斯特英语用法词典)