Usually they have an increased faction or rare spawn and a little marketplace sale around Dec 27 - Jan 3. Like a new year's thing
Found a thread from 2016 where Fanra posted some details about bonus events they started in 2014. No idea why they aren't following the same bonus schedule: 2014 15 Days of Holiday Cheer Schedule: Friday, December 12 - Double Faction & Double Rare Spawns Looking to raise your faction? Are you hunting down a rare spawn? Starting on Friday at 10AM PST, and running until Sunday, December 14 at 11:59PM PST you’ll be able to take advantage of double of both of these things! Saturday, December 13 – All Severed Hand Mounts on Sale, Before They Escape To The Vault! Starting Saturday, all of the severed hands mounts will be on sale for 400 SC. This sale will run until Saturday, December 20 at 11:59PM PST. Get them while you can. After this sale ends, these items will be retired to the Norrathian Vault, and will no longer be available as a normal item in the Marketplace. Sunday, December 14 – Experience Potions 50% Off! Shared Bottle III and Bottle of Adventure III will both be 50% off this Sunday only! Don’t miss your chance to stock up on these items. Monday, December 15 – Housing Pets 10% Off Take an extra 10% off EVERY item in the Housing/Pet category! Tuesday, December 16 – Statue Sale! Looking for a statue bust for your home? If so, take 50% off the following statue items! You can find these all in the Housing Furniture section of the Marketplace: Drakkin Male Bust Statue Large Male Bust Statue Dwarf Bust Statue Female Bust Statue Slim Male Bust Statue Drakkin Female Bust Statue Teir`Dal Female Warrior Statue Teir`Dal Male Warrior Statue Wednesday, December 17 – Hero’s Forge Armor Sets Looking to gear up in the latest Hero’s Forge fashions? Take up to 25% off ANY of the many sets currently available in the Marketplace. Look for these under Appearances/Armor Visuals. Thursday, December 18 – Big Discounts on Special Housing Items Take a HUGE discount (up to 50%!) on everything in the Housing/Special category in the Marketplace. This includes: Guild Writ of the Sunrise Door to No Where Package Primary Anchor Package Grand Guild Hall Package Palatial Guild Hall Package Grand Guild Hall and Writ Bundle Palatial Guild Hall and Writ Bundle There are more things planned for the rest of the 15 Days of Holiday Cheer coming out December 19 - December 26. But Wait, There’s More! 15 Days of Holiday Cheer Continues Friday, December 19 - Double Experience Weekend! "Starting at 10AM PST on Friday, December 19 and running until 2PM PST on Monday, December 22, 2014 get TWICE the experience you normally would as you adventure in Norrath!" Saturday, December 20 – General Services 25% Off! Saturday only take up to 25% off everything in the Services/General Category. Sunday, December 21 – Delivery Vouchers and Bayle Marks Up To 30% Off Monday, December 22 – Mercenary Slots and Character Slots 25% Off Tuesday, December 23 – XP Potions on Sale Up to 25% Off Wednesday, December 24 – Paintings Up to 50% Off Thursday, December 25 – Up to 50% Off Marketplace Items With Dragon In Their Name Take a dragon-sized discount (up to 50%!) on the following items with dragon in their name, found in the Marketplace! Dragon Sword Ornament Dragonscale Skystrider Komodo Dragon Saddle Death Dragon Shield Ornament Pot of Dragon Gold Dragonfly Saddle Shield of Dragon Gaze Ornament Friday, December 26 – Legends of Norrath Packs Save big when you purchase Legends of Norrath packs for ONLY 99 SC from Friday until Sunday, December 28 at 11:59PM PST. Also save up to 40% on Tournament Packs and Booster Boxes.
The expansion that year (2014) had already been launched when December and all of those bonuses rolled around. Ever since they started launching expansions 2 weeks before Christmas (2017), you can kiss any big bonuses (especially xp) in December/January good-bye. Maybe bonus rares and a sale, but that's about it.
