There are multiple calendars for conferences and events. It doesn’t seem possible to have a one-size-fits-all system that fits everybody’s needs. Therefore there are different solutions to cater to different requirements.
Confradar.com has a special tag for alenetwork. It lists all Agile & Lean conferences of one day or bigger on a web page, with an option to add speakers and organizers. The site is maintained by Marcin Niebudek, and anyone is invited to add conferences.
There are two generic Google calendars offered by Yves Hanoulle, the Agile Conferences Calendar and an Agile Events Calendar. Both are maintained by multiple people, and they cover conferences and events all across the world.
If you want to focus only on European conferences, then check out Jurgen Appelo’sAgile Lean Europe calendar and Agile Lean Europe Conferences spreadsheet. These are Jurgen’s personal documents which he maintains himself.