Cyber Monday is whatever Monday the Internet merchants want it to be.
There was some publicity about the Monday after Thanksgiving being Cyber Monday, but it wasn't because that was the busiest day for online shopping. Last year it was "only" the fourth or fifth busiest day. The Internet merchants just wanted to answer the Black Friday publicity.
The busiest day varies depending on who you measure it, but certainly last week, this week, and next week are all pretty busy, and that's what matters.