A post is relevant to the history of rationality if it discusses past events pertaining to the LW community or another topic related to rationality, or the post is of strong historical interest.
You can see a list of all LW posts, from oldest to newest, here.
If you're trying to get a better understanding of the community and culture here, you may also be interested in the tag Terminology/Jargon (meta).
Related Pages: History of Less Wrong, History
Historically relevant organizations, groups, and movements include: