The Seahawks official 2015 offseason start on April 20th, and consists of three phases.
Per the NFL's Press Release:
As per Article 21 of the Collective Bargaining Agreement, each club's official, voluntary nine-week offseason program is conducted in three phases:
Phase One consists of the first two weeks of the program with activities limited to strength and conditioning and physical rehabilitation only.
Phase Two consists of the next three weeks of the program. On-field workouts may include individual player instruction and drills as well as team practice conducted on a "separates" basis. No live contact or team offense vs. team defense drills are permitted.
Phase Three consists of the next four weeks of the program. Teams may conduct a total of 10 days of organized team practice activity, or "OTAs". No live contact is permitted, but 7-on-7, 9-on-7, and 11-on-11 drills are permitted.
Article 22 of the Collective Bargaining Agreement stipulates that clubs may hold one mandatory minicamp for veteran players. This minicamp, noted below, must occur during Phase Three of the offseason program.
Each club may hold a rookie football development program for a period of seven weeks, which in 2015 may begin on May 11. During this period, no activities may be held on weekends, with the exception of one post-NFL Draft rookie minicamp, which may be conducted on either the first or second weekend following the draft. The dates of the post-draft rookie minicamps will be circulated at a later date.
Seattle's OTAs take place from May 26-27, May 29, June 1-2, June 4, and June 8-11. Their mandatory minicamp is June 16th-18th.