(This is the traditional Orthodox greeting and response, respectively, at the time of the Great Feast of the Nativity of Our Lord and God and Savior Jesus Christ in the Flesh.)
Much is being made of Protestant churches that aren't having services this morning, a Sunday but Christmas. Many have had services this evening, Christmas Eve. For Orthodox (as for Judaism and other old religions), every liturgical day has always begun at sundown, not at midnight or dawn. Nevertheless, we usually have the day's Eucharistic Divine Liturgy in the morning. But for Feasts like Pascha ("Easter") and Nativity, the day's Eucharist is or may be the evening before. And we can't have more than one Eucharist on an altar in a day (unlike other churches), so an evening Eucharist is "it," unless there's a special, different kind of service the next day, like the Pascha Agape service.
Ironically, the Protestant practice mimics Orthodoxy somewhat, in marking the Feast the previous evening!