Is there a word which means "having a frequency of decades" or "per decade"? What about century and millennium?

I have a document with the headings: daily, weekly, monthly, yearly, and decadely. Google Chrome, Google Docs, and Dictionary.com insist that "decadely" is not a word. Furthermore, deacadely sounds and looks weird to me.

Is there a word I am unaware of which captures this meaning?

What would you call something that happens each century or each millennium?

I would just use "per decade" and "per century", but the words you're looking for are decennially (as in "the census is done decennially") and centennially (and millennially). See Wikipedia: Anniversary for more words.

Decennially,Centennially, and Millennially are probably the words you're looking for....