Lewis Short
Ŏpālĭa (noun N) :
* A festival celebrated on the 19th of December, in honor of the goddess Ops, Varr. L. L. 6, § 22 Müll.; cf.: Opalia dies festi, quibus supplicatur Opi, Paul. ex Fest. p. 185 Müll.; Macr. S. 1, 10.
Charlton T. Lewis, Charles Short, A Latin Dictionary