This takes place right after Zeus and Hera get married. They are on their honeymoon. Demeter is with Persephone as it is the summer, Hestia is trying to keep the peace of Olympus and ruling in the absence of Zeus and Hera. And Poseidon and Hades... Well nobody thought a God could get pregnant but it turns out they can. Because of what time it takes place all of the Gods on the Olympian Council who are not Zeus's siblings have not been born yet and Aphrodite has not yet been found, Poseidon is not yet married.