Cyber Monday is a marketing term to describe a day of discounted ecommerce trading, and comes after Thanksgiving Thursday and "Black Friday." Black Friday is one of the biggest consumer discount retail days in the US and is similar to post-Christmas Boxing day sales in the UK and St Stephens Day sales in Ireland. Coined in 2005 as a way to convince people to spend online, #CyberMonday is due to see a $1 billion splurge online.
Essentially, its a marketing gimmick - not unlike creating Valentines Day or De Beers' famous "A Diamond if forever campaign" - which created a market for diamonds that didn't previously exist!
This figure though is massively overshadowed by the recent news that Taobao, the online Chinese retailing behemoth, closed $3 billion in sales in one day!
If you haven't taken advantage of it - its probably too late. A quick Google search reveals that hundreds of online marketers already have a head start, which included securing EMD for cybermonday2012.