The 2018 Swedish general election to the Swedish Riksdag and to the municipal and county councils is scheduled for Sunday, September 9, 2018.
If you are abroad, you can vote by mail, or vote at one of the many Swedish embassies and consulates around the world. The Embassy of Sweden in Washington, D.C. will be open for voting as well as many of the Swedish Consulates along with the Swedish Church in San Pedro, CA. A complete list of the polling stations in the U.S. and the dates and times they are open is available through below link. If you have any questions, please contact the polling station where you want to go to cast your vote for more details.
The Swedish Election Authority (Valmyndigheten) will send out voting cards and material to vote by mail in July.
July 26 is the first day you can vote by mail. Any vote sent before July 26 will be invalid. Voting from abroad at for example, Embassy of Sweden in Washington, D.C., can start at earliest on August 16.
More information here