The Elections Calendar lines up with each State's .gov posted elections listings - and secondary sources get tapped, like: Ballotpedia, US Vote Foundation, among several others. Best practice is to always confirm locally - and respective links to help with that are in each calendar listing. 

Some States do not post all local elections details on their State .gov website, which can make tracking and mapping every local election difficult. The HQ keeps pace with the biggest voting organizations' elections calendars though, and this is one of the most regularly worked on pages to ensure max accuracy. You'd laugh at my bookmarks setup but don't worry, I touch plenty of grass too.

Find your local elections office.