I suppose it should really be called Federation Week as related events run from today through until downtime on the 22nd.
But Federation Day celebrates the founding of the Federated Union of Gallente Prime in 23,121 CE, some 238 years ago in the timeline of New Eden.
As has become the norm, the event will feature a five day login reward campaign featuring SKINs, and skill points, and fireworks, and a few other items.
Per the norm, Omega characters get more stuff, though Alpha characters who upgrade during the event can go back and claim items they missed.
In addition, the daily tasks during the event will bring you to Gallente space to do things like launch fireworks at the Federation Navy fleet out on parade.
There are also some new monuments that came in with the update today.
In addition there are mining blitz and Abyssal proving ground events related to Federation Day, along with some special SKINs being made available in the New Eden Store. There are also SKINs being bundled in when you purchase PLEX during the event.
All the details can be found in the dev blog post.