When is the best time to go to Guangzhou, Shenzhen and Hong Kong in a year? Give a reason to recommend?

Now the weather in Guangzhou and Shenzhen is very good, so it will be interesting to come from the north-wear short sleeves and ... (the cold air will come in a week)

Then these two months in Hong Kong are the year-end shopping festival, with many special prices (there is nothing to see in Hong Kong, only amusement parks and shopping).