Day & Night
The 3.3 version of Day & Night for Mac was available to download on the developer's website when we last checked. We cannot confirm if there is a free download of this app available. We recommend checking the downloaded files with any free antivirus. The software belongs to Theming Tools. The actual developer of this free Mac application is Brandon Roehl.
From the developer:
Day & Night changes your wallpaper based on the time of day. This app will also run any script in the scripts folder during transitions from one state to the next. Simply place any Unix, AppleScript, or Automator Workflow scripts in the scripts folder and set its name in setting.
You may want to check out more software for Mac, such as Day & Night World Map Studio, Day & Night Map or Castle Secrets: Between Day and Night, which might be similar to Day & Night.