Spring (Mar-May): the classic sweet spot
Mild everywhere, blossoms in the south, clear skies in the north. March-April: plum and peach blossom, Hangzhou's Longjing tea harvest, Luoyang's peony festival. May warms up and crowds build toward the May Day holiday (early May — book ahead).
North and central China are at their most photogenic; bring layers — mornings stay cool.