Straight: In the Magical Land of Narmnia, Aerith's family invites Bob (The Hero from the real world) to their traditional Life Day dinner. Bob finds that Life Day is similar to Christmas; it includes practices such as decorating a tree, singing melodies, hanging socks on the chimney, and receiving gifts from Father Winter.
Life Day is a winter holiday commemorating the birth of Life Dude, the festive melodies are rewrites of real Christmas carols, and Aerith's family attends a house of worship that is eerily similar to a Christian church.
Narmnia has "clones" of other Earth holidays as well as Christmas - Pumpkin Night, Heart Day, Chocolate Egg Day, etc.
Downplayed: People get together for Life Day and they eat at the same table. Other than that, there is barely any parallel between Life Day and any Earth holiday.
Christianity does exist in Narmnia, but it is forbidden by the Wicked Troll Government. Aerith's family is trying to celebrate Christmas as much as it is possible without vulnerating the actual word of the law.
Inverted: In Narmnia, "Christmas" is the month when the secret priestly initiation rituals of the Narmnian Church are performed, which are believed to be mystically associated with Christ. Their details are concealed partly through a pretense about a man named Nicholas. Other than this, the ceremonies have little to do with Christmas as we know it.
Bob, who fell on Narmnia through a magic door to our world, thinks at first that Life Day must be somewhat like Christmas. However, when Bob visits Aerith's house for the Life Day dinner, there appears to be no similarity between Christmas and Life Day.
The holiday seems similar to Christmas at first, but it turns out there is a big difference.
The people of Narmnia celebrate Christmas as well as Life Day.
...except that when dinner ends, everybody gets up to decorate a tree and hang socks on the chimney for Father Winter to fill with toys when he comes at midnight. They then walk around town singing melodies.
The holiday they call "Christmas" is not a religious holiday, and its traditions have nothing in common with Christmas as we know it. Their only Christmas-like holiday is Life Day.
Life Day's traditions combine those of Christmas and Halloween. They decorate a tree with paper ghosts and pumpkins filled with colored lights, walk around the neighborhood in costumes while singing festive melodies, and receive bags of candy from Father Winter.
Life Day is celebrated with parodies of Christmas traditions. They decorate a pile of laundry instead of a tree, burn socks in the fireplace, sing violent songs, and receive deadly weapons as gifts.
A Quirky Townin the real world observes Life Day, and may or may not even know about Christmas.
Zig Zagged: Life Day is a day to celebrate the birth of a Narmnian troll folk hero who is coincidentally named "Jesus Christ".
Christmas is celebrated in Narmnia, and is always known as Christmas.
There is a holiday special, aired close to Christmas, about a Narmnian holiday that does not resemble Christmas at all and fits with previous traditions mentioned earlier in the show.
Enforced: The writers offer the executives a lengthy and reasonable tirade about why Christianity can't exist in Narmnia. They offer to do a holiday special still, featuring a holiday that resembles Christmas, but is either a Crystal Dragon Jesus equivalent or an entirely consumerism-based holiday with no religious content. The executives agree.
Lampshaded: Bob says, "This is just like Christmas back home with my family." Aerith's kid sister asks, "What's a Christmas?"
Invoked: At first, no Christmas-like holiday is celebrated in Narmnia. However, Bob decides to invent one (maybe he's homesick), and it catches on.
Exploited: Bob uses Life Day as an excuse to sing the Christmas carols he loves, with some revisions.
Defied: Bob decides to tell Aerith about Christmas. Aerith notes its similarities to Life Day, and tells her family about it. Three years later, Christmas has completely replaced Life Day in Narmnia.
Discussed: "You should come to our Life Day dinner, Bob. My mother's holiday recipes are amazing."
"Incredible how the writers would go through such great lengths to create a completely alternate civilization only to rip off our holidays in the end."
"Ever notice how every fictional race has a Christmas-like winter holiday?"
Implied: Aerith puts up very Christmas-like decorations, and claims they're for Life Day, but we don't get to see anything else relating to Life Day.
Played For Drama: Bob is secretly a devout Christian living in Narmnia, a nightmarish Brave New World-esque dystopia where all religions are illegal. Christmas, which was once a national holiday in Narmnia, has been replaced by "Life Day", a government-approved holiday devoid of any religious meaning and entirely centered around consumerism. Following a denounce to the Narmnian secret police by Aerith, who had noticed something "odd" in Bob's behavior during the Life Day dinner, Bob is arrested. They decide to spare him the usual punishment of death, instead sending him to a re-education camp.
Plotted A Good Waste: Not applicable. (This trope is always intentionally used.)
Untwisted: On his way to Aerith's house, Bob imagines what Life Day might be like. He never guesses that it's a Christmas-like holiday, which it is.
Unparodied: Judging by the decorations at Aerith's house, Bob assumes that Life Day is a combination of Halloween and Christmas. However, it is actually a Christmas-like holiday, plain and simple. Aerith's family just hasn't taken down the Halloween (or "Pumpkin Night") decorations yet.