In the ancient world, the end of the day was when the sun went down. The night time counted as the beginning of the next day. This is the Biblical reckoning: “The evening and the morning were the first day.” To this day, Jews begin celebrating the Sabbath as soon as the sun goes down on Friday night.
This means that Christmas begins with Christmas Eve. So if your custom is to open presents on Christmas Eve, as long as you do it after the sun goes down, IT’S ALL RIGHT!