For us, it is not one particular event, but just the overall feeling you get spending the holidays in Disneyland. The holiday themed rides (Small World and Haunted Mansion) the characters dressed in their Christmas best, the holiday parade, the carolers in the Grand Californian lobby, the decorations up and down main street.
We get to spend time together as a family doing something we love, and have our own little mini Christmas in our hotel room (mini tree, no gifts since our trip is our gift--just stockings filled with little treats for us to open Christmas morning)
We can't afford to do Disneyland at Christmas every year, but we always look forward to the times we can be there.