Airports
The nearest major airport is Seattle-Tacoma International Airport (SEA), which is approximately:
        
However, another option is Bellingham International Airport (BLI), which is approximately:

Traffic
The Seattle metro area is known for traffic congestion, so expect delays when heading northbound from Seattle-Tacoma airport on Friday afternoon. You will be able to take advantage of express lanes (one-way northbound through downtown Seattle in the afternoon), and carpool lanes. There will be minimal congestion on Saturday morning, Sunday morning, and Sunday afternoon. Here are some handy links from the Washington Department of Transportation regarding real-time travel times and traffic conditions.

Ferries
The Puget Sound region is famous for Washington State Ferries, and if you are looking for a unique travel opportunity, you may choose to take the sailing between the Mukilteo and Clinton terminals en route from Seattle to Camp Casey Conference Center on Whidbey Island. Sailings depart both terminals every 30 minutes at on the hour and half past (schedule here), and the crossing takes about 20 minutes. Fares for vehicles (including driver) are between $7-8 each way, and fares for additional passengers are between $2-5 north bound and are free south bound. See this calculator for exact fares, which change seasonally. Peak travel volumes occur during the afternoon commute (3-8pm), so there may be a wait to leave the Mukilteo terminal on Friday afternoon. You can check here for real-time traffic cameras. There should be minimal waits northbound on Saturday morning and southbound on Sunday morning.

You are not required to take a ferry to visit Whidbey Island from Seattle, as there is a bridge across Deception Pass that connects the north end of the island with the mainland. This route option adds approximately:
However, depending on the ferry schedule, wait times, and highway traffic, this route may or may not be faster. If you are coming from the north, for example Bellingham International Airport, you will definitely choose this route.

Event Parking & Shuttles