In the USA, we like shopping. After Thanksgiving we have Black Friday, Small Business Saturday, Charity Sunday (AKA White Sunday), and finally, Cyber Monday. Cyber Monday is when we shop the sales on-line.
This is a bit silly. Most large retailers, including online retailers like Amazon, hold Black Friday sales just like regular stores... well, not just like; this year Amazon held a 'Black Friday Deals Week'. But not to worry, they'll be having a Cyber Monday sale as well.
The term Cyber Monday was coined in 2005 by Ellen Davis, the senior vice president of the National Retail Federation. It was coined after the fact, as marketers noticed that the Monday after Thanksgiving was one of the biggest online shopping days of the year (the days preceding Christmas top it by a bit). So they hyped it, and it caught on.
Cyber Monday continues to grow; while the trend is uneven, online retailers are seeing about a 15% increase in Cyber Monday sales each year over the last nine years. This may be driven by a psychological reaction in the general form "remember how fun shopping was? Isn't work a pain? I deserve more shopping to ease my pain". Whatever is causing it, it is a big deal, and we are likely to keep hearing lots of Cyber Monday hype for the foreseeable future.