VeHayita Ach Sameach

Chazal say that "ach sameach - specifcally happy" applies to the last day of Sukkos. Rav S.Y. Zevin suggests that what's unique about the last day of Sukkos is that it has the potential of being a time of pure joy. Like Olam Haba pure. As opposed to usual joy that includes sadness and frustration as part of its process.