Why do we celebrate Christmas Eve?

If you think about it, it's the day before Christmas, so it's good to celebrate because you know tomorrow is an exciting day. Also, if you think about it, why do we celebrate New Year's Eve? Because you know it is the last day of whatever year it is, and a new year is yet to come. It's the same, and many young children like to celebrate by putting their sacks out for Santa Claus.