"J-day" might not make much sense to you but to all Danes, especially the youth, it does! The "J" stands for "Juleøl" (Christmas-brew) and J-day is the day that Tuborg releases their infamous christmas edition. Yes, it's the same regular pilsner every year, but who doesn't like an excuse to go crazy and get drunk?
This tradition didn't start until Tuborg announced the first J-day in 1990 as a part of a marketing campaign but never the less it's a huge event today, especially in Copenhagen where large trucks spread white foam ('snow') on a lot of big squares. The Danish police even arranges special drunk-driving controls on this day.
J-day officially falls on the first friday of November.