There does not appear to be a button trigger for the xmas hidden event. I spent some time looking around in the game and I have everything unlocked except for it. Then I started digging in the code to see if I could find it. I can find references to if it has been seen or not, but have not found a specific trigger for it yet.
I did find in the initialization scripts where it is specifically hiding the triggers for the other three hidden events in the scripts, but no mention of the xmas event.
renpy.hide_screen("swm_cross")
renpy.hide_screen("game_show_button")
renpy.hide_screen("sleep_day_button")
If you have developer mode enabled, you can force the event to show in the hidden events menu by typing in the following command and then go through it by "replaying" it from that menu, but since you cheated it in and did not use the trigger it will still show 3/4 hidden events found.
"finished_xmas_story = True"
capitalization matters.