The rally, hosted by the Seahawks and Seattle Center, will feature former players, members of the Sea Gals cheerleading squad, Blue Thunder, Blitz, and Mayor Ed Murray.
Fans will also be able to sign the 25'x35' flag that's flown atop the Space Needle during the playoffs and in previous years.
The flag will then be shipped to New York, where it will be taken around the city for fans to sign before it is presented to the team Sunday at MetLife Stadium in New Jersey.
"Seattle relishes opportunities like this to show our civic pride," said Space Needle spokesman Sean Marshall. "The very first 12th Man flag flew atop the Space Needle in 2005. It's captured the imagination and the spirit of the city quite like no other. It's exciting for us to have this shared history and it just makes sense to try to do something big to help the team."
Plans are underway to unfurl the flag in a number of iconic spots including Central Park, Marshall says.
Updates from this journey of the "12" Flag will be announced every step of the way via Space Needle social channels.
The rally will run from 11:15 a.m. to 12:30 p.m. at the Fisher Pavilion and South Fountain Lawn at Seattle Center. The flag will be on display and available for signing from 12:30-3:30 p.m.
Seattle Center will offer free round-trip transportation on the Monorail from 10:00 a.m. to 3 p.m. for all fans wearing Seahawks apparel.