The short answer: not many places.

Courtesy: YFD

The City of Yakima Fire Department is again reminding its citizens about the dangers of, and the ban on, fireworks.
Each year, the Yakima Fire Department responds to multiple incidents caused by illegal fireworks use. Since 1982, the City of Yakima has had an ordinance banning use of fireworks within the City, as it is can under the Revised Code of Washington.

There are, however, a few towns within Yakima County that have less restrictive laws.

The three municipalities withing Yakima County that do allow for the legal sale and/or discharge of fireworks - with some restrictions - are Mabton, Moxee, and Zillah.

Washington State Fire Marshal