As I understand it, the answer to a mystery of today's protest schedule is this:

The Seattle Port Shutdown forces plan to meet at a Port of Seattle entrance near the intersection of SW Spokane and SW Manning, on Harbor Island, at 3 p.m. this afternoon because that's when a big shift change at the port begins.

The idea is that by creating a picket line at this entrance at 3 p.m., the protesters will force union port workers to consider whether they can cross said picket line, and if the answer is "no"—well, presto, port shutdown.