Wash. Rev. Code § 35.16.010
Upon the filing of a petition which is sufficient as determined by RCW 35A.01.040 requesting the exclusion from the boundaries of a city or town of an area described by metes and bounds or by reference to a recorded plat or government survey, signed by qualified voters of the city or town equal in number to not less than ten percent of the number of voters voting at the last general municipal election, the city or town legislative body shall submit the question to the voters. As an alternate method, the legislative body of the city or town may by resolution submit a proposal to the voters for excluding such a described area from the boundaries of the city or town. The question shall be submitted at the next general municipal election if one is to be held within one hundred eighty days or at a special election called for that purpose not less than ninety days nor more than one hundred eighty days after the certification of sufficiency of the petition or the passage of the resolution. The petition or resolution shall set out and describe the territory to be excluded from the city or town, together with the boundaries of the city or town as it will exist after such change is made.
RCW 35.16.010
Times for holding elections: RCW 29A.04.311 through 29A.04.330.