From the Organizers:
Yuri’s Night is a global celebration of humanity’s past, present, and future in space. Yuri’s Night parties and events are held around the world every April in commemoration of April 12, 1961, the day of cosmonaut Yuri Gagarin’s first manned spaceflight, and April 12, 1981, the inaugural launch of NASA’s Space Shuttle.
Yuri’s Night events combine space-themed partying with education and outreach. These events can range from an all-night mix of techno and technology at a NASA Center, to a movie showing and stargazing at your local college, to a gathering of friends at a bar or barbecue.
In 2011, the 50th anniversary of human spaceflight, over 100,000 people attended 567 officially-recognized events in 75 countries on all 7 continents, while tens of thousands more watched the 12-hour live Yuri’s Night Global Webcast and participated online in the virtual world of Second Life.
For more details on the Yuri’s Night, take a look at the official website: yurisnight.net.
To learn more about Yuri Gagarin, try his Wikipedia Entry: http://en.wikipedia.org/wiki/Yuri_Gagarin