I'm interested in marriage/divorce information by birth date.
Does anyone know the best place to get something like that?
Something like this?
You can find the spreadsheet on the bottom of the page.
There's are a number of studies on various 'birthday effects' in economics literature. I don't know of any that use open data, but you could contact the authors for help in your research.
Here's an example from the genre. http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2241677