When did the Chinese start to switch over from writing in literary Chinese to writing in spoken Chinese?

ANSWER: Sometime around May 4, 1919.

The May Fourth Movement in China, which started with protests in Beijing on May 4, 1919, is generally regarded as when the Chinese began to switch over from writing in literary Chinese to writing in spoken Chinese. The difference between the two is essentially that to understand something written in literary Chinese people would need to see and read the Chinese characters themselves, but to understand something written in spoken Chinese, people could either read it themselves or just listen to someone read it to them. So, after the Chinese started writing in spoken Chinese, even illiterate people could enjoy newspaper articles and novels when those newspaper articles and novels were read to them.

The Chinese slogan that was used at the time to describe writing in spoken Chinese was 我手寫我口 (아수사아구), which literally means "My (我) hand (手) writes (寫) my (我) mouth (口)." I mention this because this morning I came across the following song that uses that old Chinese slogan as its theme.