“Because a Christian lithurgical day always starts and ends at sunset. In Southern Scandinavia, the lithurgical day on which Christ was born by tradition starts at 5 PM the 24th and ends on 5 PM the 25th. Celebrating Christmas on the 25th would imply having Christmas dinner the day after the event, as the lithurgical day ends on sunset the 25th. Christmas eve is therefore celebrated on the 24th while Christmas morning is celebrated on the 25th” (Sturla Molden, Quora).