In the US, Black Friday is the busiest shopping day of the year so American retailers discounted on that day. As they went into other countries, they began offering sales on that day too. In the UK, Boxing Day was traditionally the busiest shopping day of the year but most shops close on this day now. Wikipedia tells me that Boxing Day used to be the busiest shopping day in Canada.
I think the reason the practice is spreading into the rest of the month is because of the higher level of competition online. One retailer does Black Friday, then another retailer tries to jump earlier, then another. In the UK, we have had companies trying weekly and monthly sales.
The sales aren't bad, I think some retailers (Amazon, particularly) don't offer great value...ironic given they started Black Friday here in the UK...but if you hunt about, you can usually find some decent deals.