The Collaborative International Dictionary
Vacuna \Va*cu"na\, n. [L. vacuus unoccupied.] (Rom. Myth.) The goddess of rural leisure, to whom the husbandmen sacrificed at the close of the harvest. She was especially honored by the Sabines.
Wikipedia
Vacuna was an ancient Sabine goddess, identified by ancient Roman sources and later scholars with numerous other goddesses, including Ceres, Diana, Nike, Minerva, Bellona, Venus and Victoria. She was mainly worshipped at a sanctuary near Horace's villa (now in the commune of Licenza), in sacred woods at Reate, and at Rome.
The protection she was asked to provide remains obscure. Pomponius Porphyrion calls her incerta specie (of an uncertain kind) in his commentaries on Horace. Renaissance authors and Leonhard Schmitz state that she was a divinity to whom the country people offered sacrifices when the labours of the field were over, that is, when they were at leisure, vacui.
The etymology of her name is linked to lack and privation, and Horace appears to call upon her in favour of a friend to whom one of his epistles is addressed. From this, it has been conjectured that she was prayed to in favour of absent people, family members or friends.