http://revach.net/article.php?id=4108

Section: Moadim   Category: Succos
Shmini Atzeres: The Day The Tefilos Will Finally Arrive
Shmini Atzeres is the last day of Yom Tov but it is a Yom Tov by itself.  It is different from Succos in six things and the gemara lists them and give an acronym by which to remember them, "Pzar Kshav".  We also know that Shmini Atzeres is a very special day for tefila when all our tefilos are answered.

The Sar Shalom of Belz says that we see this in the words spelled out by the acronym of the days uniqueness.  "Pzar" means scattered and "Kshav" means to listen.  Not only does Hashem listen to all our tefilos on Shmini Atzeres, but it is also the day that He responds to all our previous tefilos that had gone unanswered.  "Pzar", all the tefilos that were scattered and never reached their destination.  "Kshav", Hashem will listen to then as well.