See where to vote: http://www.sec.state.ma.us/wheredoivotema/bal/MyElectionInfo.aspx
Here's an overview from the Waltham News Tribune on the 5 candidates: http://waltham.wickedlocal.com/news/20180731/voters-in-walthams-ward-6-going-to-polls-in-preliminary-election. This preliminary election will narrow down the final selection to two candidates.
The municipal election for Ward 6 will be held on September 25, 2018. The last day to register to vote for the Ward 6 Municipal Election is September 5th. See the full election schedule here: https://www.city.waltham.ma.us/sites/walthamma/files/uploads/special_municipal_election_0.pdf