Do you mean "observance"?

"observence" is a common misspelling of "observance"

How to remember the correct way to spell "observance":

An exception to the rule that most words ending in 'e' like 'observe' take 'ence' not 'ance'.

Test your Spelling • Common Misspellings • Terms of Use • Top Ten Misspellings • Contact DoYouMean