Hestia gave up her seat as an Olympian to Dionysus.
Rick Riordan named his fifth book The Last Olympian because the there was only one Olympian left on Olympus after the rest of the gods left to go fight. The last Olympian on Olympus was Lady Hestia or to the Romans Lady Vesta.
The Olympian gods met on Mt. Olympus
mount Olympus
Olympian means people who live in Olympus like Zeus,Poseidon and etc.
Mount Olympus
"Heroes of Olympus" takes place about six months after the events of "The Last Olympian."
Olympian Athena likely spent most of her time upon Mount Olympus, or above it. Mount Olympus was dwelling place of the Olympian gods.
Chronos of course xD
As Apollo is one of the Twelve Olympian ruling gods; he was welcome on Mt. Olympus.
He is not Olympian, Hades dwells in the Underworld by preference rather then the lofty heights of Olympus.
Hestia did.When there were no seats left for the young Olympian,Hestia gave up her seat for him.That's why he sits on the women's side of Olympus.
She is Olympian, not from Olympia. Her residence is on Mount Olympus.