The real world book day is on 23rd April, but the UK has its book day in March (mainly for commercial reasons, as otherwise it is a rather flat time for book sales) and rather grandiously calls it "World Book Day". The "Day" was on the 3rd March, but tonight (5th March) is "World Book Night" and there are various celebrations (readings, special TV programmes etc.) as well as books being given away. The website is at http://www.worldbookday.com/ and includes some information on setting up reading groups, inviting authors to schools etc.
Photo by Sheila Webber: Early blossom, Hailsham, March 2011