The trouble is: search engines don't really like you trying to look for date ranges. A search term like "technology conference march 2006" doesn't really mean anything to Google. You'll get lots of results, but nothing to help you order results by date or by location.

An alternative approach is to search for a search engine that might do what you want. For instance, "conference search engine" turned up...

http://www.allconferences.com/

Now if only its advanced search didn't seem to behave in a random manner... :-)