American Dad launched its 15th season with a holiday special that aired on Christmas day. This makes it one of the longest-running shows on television. Not many shows make it this far, American Dad is in a class with the Simpsons, South Park, and Family Guy despite being very underrated.
The rest of Season 15 will run as a weekly series as it always has, starting on February 15th on TBS. Since moving to TBS, from Fox, American Dad has been able to retain most of its very loyal fanbase.
If you missed the Christmas special you should watch it on Hulu. Don't be turned off by "Ninety North, Zero West", the previous Christmas special, this one was a lot better and focused mostly on Roger.
Make sure you catch the new season either on TBS or Hulu.