When I was a kid, everything was opened Christmas morning and Christmas Day as we visited various relatives.
When dh was a kid, they opened everything (even stuff from Santa) on Christmas Eve. They had a tradition that his mom would take them for a walk on the beach in the early evening, and Santa would come while they were walking. My MIL was German/Swiss, and perhaps Christmas Eve gift opening is a tradition in Germany or Switzerland??
For our family, we used to open MIL's gifts on Christmas Eve because that was her tradition and everything else on Christmas Day, but MIL is no longer with us. Now, we open everything Christmas morning. Our kids no longer "do Santa," be we still wait until they go to bed to put the presents under the tree. We enjoy the anticipation and "wow" factor of getting up in the morning and seeing the presents under the tree.