The holiday of Succot has two special Mitzvot: Lulav and Succa. Each of them have the blessing of שהחיינו attached to it. We express extreme gratitude to Hashem for allowing us to live another year to observe these Mitzvot.
We generally say the שהחיינו the first time we sit in the Succa and the first time one makes the Bracha on the Lulav and Etrog. If one forgets to make the שהחיינו, he can do so all week long.
The Mitzva of Succa is unique. That is, if one were experiencing intense joy while building the Succa, he can say the שהחיינו even before Succot. He is thanking Hashem for the opportunity to do the Mitzva of Succa. He must remember that his שהחיינו for Succa is covered during construction. He does not make another שהחיינו on the first night of Succot when he sits in the Succa. One שהחיינו for each particular Mitzva.