It is indeed pretty rare. Not counting Sunday's 81, it has reached 80 or above just 10 times in the past 48 years, so it happens roughly once every other year. The last time was an 81 degree day on April 16, 1999, so we were due.
The 78-degree day on March 29 was perhaps more impressive. It set the all-time record high for March at Sea-Tac Airport ( the Federal Building in downtown Seattle did report an 81 degree reading on March 11, 1892.) So those occur roughly once every century.