May in China - Weather and Event Guide

May Overview:

May is my favorite month in China. You can pretty much depend on it being warm and balmy but the heat and humidity of the summer have not yet set in. The country has greened up and the flowers are in full bloom. You often get some clear blue skies in the major cities and there's just an overall good feeling in the air.
Northern China, such as Beijing, is beautiful. You'll be relatively dry and the temps are lovely. Across central and southern China it will still be damp, but the temps have warmed to such a degree that it won't feel uncomfortable. You'll still need your rain gear, but you'll see a lot of nice days.

May Weather:
Beijing: the average daytime temp is 80F (26C), the average number of rainy days is 6.
Harbin: the average daytime temp is 70F (21C), the average number of rainy days is 10.
Xi'An: the average daytime temp is 67F (20C), the average number of rainy days is 10.
Shanghai: the average daytime temp is 74F (23C), the average number of rainy days is 15.
Guangzhou: the average daytime temp is 85F (30C), the average number of rainy days is 19.
Guilin: the average daytime temp is 72F (22C), the average number of rainy days is 20.

See charts for Ave Temperatures in Major Cities
See charts for Ave # of Rainy Days/Month
May Packing Suggestions:

Finally, the layers can be lightened a bit.
North: very comfortable during the day. You still might need a longer layer for cooler eveings.
Central: it'll be damp but warm. Bring layers if you get cold in air-conditioning but you won't need anything too warm or heavy.
South: as with April, it'll be warm during the day, even hot, and humid. Bring light clothes that will feel comfortable under rain gear. You may want a heavier layer at night, but light pants will probably be the warmest you'll need.
Visiting China in May - Pros:
Like I said above, May is a great time to visit China for really comfortable temperatures.
May is also good as you'll avoid the big summer travel season.
Visiting China in May - Cons:
Labor Day (or May Day) used to be a week-long holiday, but with just one day off now, it shouldn't be a big problem to travel during that time. You may find larger crowds as tourist sights and worse traffic.
Rain across most of China except the North and far West. But that's just how it goes. You can't avoid rain if you're going to see the country so pack an umbrella. You'll be fine!