What date should I use for graduation?
You should use the date conferred. Universities will have an official date where all degrees for the term are conferred on everyone. This is usually not the same date as the graduation ceremony as there can be multiple graduation ceremonies across multiple days, but trying to keep track of all of them would be confusing. It's much simpler to just pick one day after all the ceremonies and say all degrees are official as of that date.

That being said, I've never heard of an employer (or anyone) who really cares about the particular date as long as it's clear that you've already met all of the requirements of graduation.