I always find out about the best conferences too late, I find http://lanyrd.com isn't that great for design listings, but perhaps there are other sites. Would love to hear people's suggestions though.
The interaction-design.org calendar is another pretty good source.
I'm going to Lean UX NYC in April. I've had a blast there the last couple of years, but (depending on your definitions) it's more product management / UX / startup than design.
I'll also be at UX Lisbon in June, which looks like it might be fun (apart from my session – that'll be rubbish ;-)
The Fetch http://thefetch.com/ lists events by city. It's not design-specific, but they have a section called "Events for Creatives." They've not been active for a while, but should be relaunching soon, after a successful Kickstarter campaign.
I've started to put conference twitter feeds together in a list, but haven't gotten too far with it.
Check out UX Munich: http://www.uxmunich.com.