Day & Night
Day & Night 3.3 for Mac is available as a free download on our software library. Our built-in antivirus checked this Mac download and rated it as 100% safe. The software is included in Theming Tools. This free software for Mac OS X is an intellectual property of 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, Castle Secrets: Between Day and Night or Amelia and Terror of the Night LITE, which might be similar to Day & Night.