Pokemon Go is having a "Spring Bloom" mini-event this weekend, which will significantly increase spawns of Grass-type Pokemon. To help promote the event and give fans a reason to drop a little bit of money into the game this weekend, Pokemon Go will increase the length that a Lure Module will work on a Poke Stop.
Lure Modules will cause Pokemon to automatically spawn around a Poke Stop, usually for a span of about thirty minutes. However, Lure Modules will work for 6 hours this weekend, meaning that you can place a single Lure and use it for most of the day. Because of the mini-event, the Lures should attract mainly Grass-type Pokemon instead of Pokemon that would usually spawn near the Poke Stop.
Right now, the only two "official" parts of the event are the increased Grass-type Pokemon spawns and the increased length in how long Lures work. There is a lot of rampant speculation, however, about the possibility that the event could be a way to sneak in more Shiny Pokemon into the game.
The new event should begin around 1 PM (Pacific Time), so keep watching WWG for updates.