I797 expiry is on 31 DEC 2018

My petition got approved on August 14th 2018, but my I797 expiry date is Dec 31st 2018. I am a bit confused as I will be going to schedule my interview some time in October 2018 and by the time I arrive I will be left with very few days of validity. Do I need to file for an extension in this case and if I need to, can I do it before going there?? And also what are the chances of getting the extension and processing time associated with it, as I need to plan whether to take my dependent during the time I have filed for an extension or not. Please guide me on this as I am getting to see so many puzzling answers on Google. Any help is highly appreciated.