The ceremony beings at 6:00, so I'd arrive 15 - 30 minutes early, depending on how much mingling you want to do.
We'd describe attire as being cocktail. We will also have a coat check for anyone who needs it.
Yes, there will be valet parking at the Four Seasons.
If you're coming from another hotel, you're on your own for transportation. Cabs and Lyfts/Ubers are everywhere, or you can coordinate directly with Sal (see my note under the "Travel" section).
Potentially...but we're still figuring this out. Likely any other events will come via a Paperless Post into your email Inbox :)
Potentially...weather in December is typically pretty mild. And by mild, I mean little to no snow; lows in the high 20's to low 30's and high's in the...30's? So yes, if you're from California, it'll be cold. Not to worry, keep your eye on the weather and plan accordingly. i.e. if snow is in the forecast, I'd recommend bringing a pair of boots. Day of the wedding, you can cab everywhere and tough out any terrain in heels. Chicago's also great about salting the sidewalks and such but that can also stain shoes, so be careful!