It's Always Sunny in Philadelphia
A full list of special holiday episodes

A darkly hilarious and irreverent comedy about a group of dysfunctional friends who own a dive bar in Philly.
The show aired 1 Christmas episode, 1 Thanksgiving episode, and 3 Halloween episodes.
Christmas episodes:

Season 6, Episode 13
A Very Sunny Christmas
Dennis and Dee take inspiration from "A Christmas Carol" to teach Frank a lesson, while Mac and Charlie discover unsettling truths about their Christmas traditions from their childhood.
Thanksgiving episodes:

Season 9, Episode 10
The Gang Squashes Their Beefs
The gang's Thanksgiving plans are ruined by their past conflicts with old enemies. In an effort to make amends, they decide to reconcile with those they have wronged.
Halloween episodes:

Season 6, Episode 7
Who Got Dee Pregnant?
The gang panics when Dee reveals she's pregnant and they can't remember who the father might be from their previous Halloween party.

Season 8, Episode 3
The Maureen Ponderosa Wedding Massacre
The gang becomes the primary suspects in a massacre that occurs at the McPoyle-Ponderosa wedding.

Season 14, Episode 6
The Janitor Always Mops Twice
Charlie finds himself in a black-and-white world, reliving a noir-style memory where Frank has been poisoned with diarrhea. With Frank out of commission, it's up to Charlie to navigate the seedy underbelly of Philadelphia.