Will I get this in december as well, If I buy over 100$ in November ? Does Sony do this every month, because I think this is the first month that I spent so much in.
The promotion was actually intended to get each customer to spend up to $100 or more on their purchases during the month of October. To answer the question though, there won't be another deal like this one in November of this year. By comparison, last year Sony threw the same kind of event for those who spent $60 through the month of November 2011. PlayStation customers will likely see the same if not a revised version of the promotion sometime next year.
Promotional news can come from various news sources. If you were to watch the PlayStation Blog, for instance, you'd find the information on any future deals posted there and then correlated spots across PlayStation-related gaming media. If you also happen to pay attention to the PlayStation Store on a regular basis then that's another way to find an advertisement or something that draws your attention to a related deal in progress. Heck, you might even have a friend or someone who takes heed of these things and would keep you in the loop when they happen if you aren't equipped to do it all yourself.
Just to add on to what Gray said, you can also keep an eye on the "What's New" section right from the XMB. That would be the most convenient way to go about finding out.