It's organized by the guilds of Zürich (mean tongues argue they still would have too much influence in the social life of the city - others say the celebration of Sechseläuten would be a employment-creating measure for the otherwise unemployed guilds). There will be a parade of the guilds in historical costumes and later the day the Böögg (a figure of a snowman symbolizing winter) will be burned. Traditionally the speed of it's burning indicates the weather of the summer.
Some more information:
the official site with fotos of the event.
As every year I completely forgot I would have free this afternoon.
Addendum: here you'll find some fotos of the event.