If you want to pick a single week to play TF2 every year, this is the week. Huge updates almost every year.
Our oldest landmark patch yet sneaks in this timeframe; the last patch of the original Quake Team Fortress (which had moved over to QuakeWorld at this point) was released in December 22, 1996.
Have you wondered why some friendly soldiers and demomen will start shooting themselves while you heal them during setup time? Obviously in regular play this can help you build Uber, but the Uber rate cut at >142.5% doesn’t apply to setup times, right? Well, it used to, back before the December 20, 2007 patch. This patch added a few other notable changes: Sudden Death became a server option (that everyone immediately stopped using), sapped buildings gained a slight damage resistance to the spy who placed the sapper, and Dustbowl no longer has that griefable teleporter spot behind the fence. It also marked the removal of the Civilian from the game, the true tenth class of TF2. Before this, you could join a server as him using the console. RIP in piece.
2009 brought us the WAR! Update! Among all the famous hullabaloo like weapons, maps, and TF2’s largest frag contest ever, this update added item crafting, in-combat damage numbers and auto-Medic calls for the first time. The Camera Beard also got its own recipe on December 22nd that let you craft it into scrap metal; this was in response to a hilarious bug where it was dropping for a while at the same rate as weapons. Even today, there are more Vintage Camera Beards than Unique ones.
December 17, 2010 saw Australian Christmas released. This is the first major TF2 update that I personally participated in. Medieval Mode became a thing, as well as my beloved Crusader’s Crossbow. The second set of item sets with set bonuses were released; these did not require the hat because people were really, really mad about that part of the Polycount sets. Itemtest was also added so people can see all of these new cosmetics; it had limited functionality and to this day doesn’t work very well. Backpack expanders were added, also in response to the increasing amount of things in TF2 to collect. My then-girlfriend now-wife found a Larrkin Robin on the very first day of the update.
On December 22nd a Stocking Stuffer Key was given to every player, so they could open a crate. This is the only time that happened.
The December 15, 2011 Australian Christmas did a lot of the same things; new weapons, festives, and the map cp_foundry, the first map to have its own set of achievements. One of them was “Win 142 rounds,” a tolerably annoying achievement which was manageable since it was the only one of its type. I played months of solid Foundry to get it. Sadly, it turns out to have set a precedent for new maps…The first strange cosmetic, the Spirit of Giving, came into existence on December 21, 2011. December 19th added the Something Special for Someone Special to the game files, though it didn’t have any data attached, it was just a placeholder, so the whole community spent two months guessing what it could be. Nobody guessed right.
December 20, 2012 brought the Mecha Update, also called Australian Christmas 2012 because that’s basically what it replaced. Mainly a Mann Vs. Machine-themed update, engineer robots and mvm_bigrock became a thing, as well as three new weapons for the classes most starved of unlocks; Medic, explosive Demoman and Engineer. These were the only new weapons released in 2012 besides the Cozy Camper and, as of this writing, the newest non-reskins added to TF2.
December 20, 2013 brought the most recent Australian Smissmas, as it is now referred to. Anyone quick enough to log in within an hour of the update going live was treated to a number of mysterious presents appearing in their backpack, filled mostly with Mann Co. Crates. I personally sent somebody a Geniune Ham Shank, hope Diegog07 liked it. No new weapons, breaking a Smissmas tradition, but the bugfixes turned the Soda Popper, Short Circuit, and Diamondback into basically new weapons. Finally it passed out name tags, description tags, and backpack expanders to everybody in TF2, crashing all three trade economies, which in my personal opinion is a 100% positive result.