Our culture is almost too far gone for "Merry Christmas" to be meaningful. In the public mind, Christmas has morphed into Decemberween, and "Decemberween is not about getting people presents. It's about getting people good presents!" So when the clerk at a store, or the HR department at work, wishes you "Happy Hollowdays," there is no point in in asking which hollow day they mean; they mean Decemberween.
Those of us who observe the Feast of the Nativity on December 25 should probably stick with the Eastern traditional greeting, "Christ is born!", to which the correct response is, "Glorify Him!"