Sam, I think it's more of a "local" reference to the fact that Kirkland, WA sits on the east side of Lake Washington near Seattle.
David is correct. Bellevue, Kirkland, Redmond, Issaquah, Medina, Clyde Hill, etc. are all "on the eastside" due to their location across from Seattle on the east side of Lake Washington.
Sam
If you are ever in Seattle in the summer let me know and we will try and arrange the deluxe three rail boat tour. (Not to be confused with that Hawaiian three hour tour. ) Our vessel departs from Seattle and crosses Lake Washington with stops in Kirkland and Medina to see Eastside Trains and the Northwest Trunk Lines respectively. In the Seattle area all of our finest O gauge attractions are only a 2 block walk from a public dock!
The fist week of August if fantastically fun but a little crazy in Seattle. The first Sunday in August is the annual Seafair unlimited hydroplane race.
Earlier in the week the Blue Angels arrive. The race course is the center point for their air show. While the course is well south of Kirkland the Blue Angels make several fly byes over Eastside Trains.
Highway traffic in the Seattle area is always horrible. It gets worse when the Blue angels are in town. The I-90 floating bridge is closed during the Blue Angels show for safety's sake.
But as congested as the traffic is on land the worst traffic jams are on the lake. Thousands of boats are out when the Blue Angels are flying!
There is plenty of fun on the lake in the summer. But there is fun to be had on Lake Washington this time of the year too. Is anyone going to Eastside Trains by Christmas Ship to pick up their Big Boy?