Let a thousand flowers bloom is a common misquotation of Chairman Mao Zedong's "Let a hundred flowers blossom".

That slogan was used in the summer of 1957 when the Chinese intelligentsia were invited to criticise the political system then obtaining in Communist China.

The full quotation, taken from a speech of Mao's in Peking in February 1957, is:

"Letting a hundred flowers blossom and a hundred schools of thought contend is the policy for promoting progress in the arts and the sciences and a flourishing socialist culture in our land."