Sword Art Online is one of the most watched anime in recent years. The series was airing its third season which is based on the Sword Art Online: Alicization arc. This arc is the most extended story arc to date. In the author’s afterword in light novel Volume 18, it was revealed that Kawahara had written the entire story for Alicization in web novel format. He started in January 2005 and ended in 2008.
After some years, Kawahara wrote a web novel called Cradle Of The Moon. It is an Underworld side story which takes place before the ending of Alicization. Sword Art Online Alicization story was adapted into light novel volumes starting from vol. 9 to 18. SAO’s later cours will most likely consist of Volumes 19 and 20, as they were called Moon Cradle part 1 and 2.
The third season of the anime will adapt the entire Alicization arc. Alicization is going to be a split-cour anime. The third season is going to have four cours. So, there are going to be over 40 episodes at least.
The third season will go on hiatus in April 2019. The second part will start in October 2019. There is no exact release date for Sword Art Online: Alicization War of Underworld. The finale of Sword Art Online season 3 should come out in March 2020. I would like to take this opportunity to clear up one significant doubt as well. There is the fourth season of Sword Art Online. The third season merely is divided into parts.
Updated on 5 July 2019:
The show is currently on a hiatus for the third season, which started by the end of March 2019. The third season is supposed to have 48 episodes, without considering special episodes. Nonetheless, the next 24 episodes will be a part of the third season itself, just to clear that the fourth season is still not around.
However, if you want to know about the fourth season, there is not much available. Nevertheless, there will be a fourth season for sure, as the show will not complete soon. If the fourth season gets a yes nod, it might be released in October 2020. It fits perfectly as the third season premiered in October 2018, and it will resume in October 2019.