GNU Manifesto published: 1985
(edit: he did that, by the way, because he had had access to the source code of their photocopier at MIT, and the company decided to stop providing that source, which was all the rage in 1985 as Microsoft bootstrapped capitalism, rather than the weird corporate hippiedom of the 70s, as the way of technology.)
Open Source Initiative formed: 1998. There was a manifesto of sorts around 1996 that I recall being the first time I heard people start to use Open Source instead of Free Software, and mash up the two into a single meme, when one them is completely about amputating the ethics of the other.