So far, for the time I've been on Manifold, there have been four markets that felt like sitewide events:

My perception of whether something feels like an event is mainly based on whether it receives a large number of traders and a high mana volume and is talked about by pretty much everyone active on the site for a while, to the point where it basically feels like that market is the main thing on the site for a short period of time. This is especially true if people say things like, "Oh, you weren't here for market X," to users who joined the site after the market resolved or stopped being as relevant. 