International Open Data Day is the last Saturday in February,
which makes it February 22, 2014
The main page http://opendataday.org/
and the wiki where everything is shared http://wiki.opendataday.org/
have not yet been updated.
You can post to the mailing list if you have questions, however I'm told the mailing list is currently being overwhelmed with spam signups. I suggest you contact OKFN or David Eaves if you want to volunteer or have ideas for better handling the mailing list spam.
I don't think there's been a hashtag declared, it will probably be #ODHD14 (Open Data Hack Day 2014)
UPDATE 2013-12-13: You can also contribute on GitHub https://github.com/okfn/opendataday.org
UPDATE 2014-01-29: Hashtag looks to be #ODD14