So many Chinese. It's called Spring Festival, like Christmas. Basically the festival last for 1 week(God I'd SOOOOO love it if it has 2-4 weeks vacation).
Back to the topic, the post office is not closed. It's just with 1.38 billion people went crazy at the same time, piles of goods need to be loaded and delivered, and most delivery men are enjoying their vacation at the mean time, delaying is common. But I doubt if it has to be 2-3 weeks. As I know, companies like TNT, FEDEX, UPS have agencies or branches in major cities, these carriers works just fine during the festival, just at higher price compare to local ones.