Extended episodes are what you can make, you'll know that it is extended if it keeps going on and on. If it is extended and the episode that you are writing is not when it airs on tv, it does not matter. You can make any episodes extended, but it is usually made for episodes such as series premieres, season premieres, season finales, and series finales.
Please write it as this to notify people if it is extended -
The following episode is extended or the following episode is extended. Either way, notify it, don't just write extended.