Century Cakes are UNCOMMON items received from Simon or the Incredible Raffle Box during the entire SkyBlock Year 100, Year 200, Year 300, and Year 400. They are placeable items which give small Stat bonuses when consumed.
Similarly to the Crab Hat of Celebration, one random type of Century Cake is given out to each player. Also like the Crab Hat of Celebration, only 1 can be claimed per account every 100 SkyBlock years. There are 14 different types, each giving a different stat bonus for 48 hours. These 48 hours include offline time, similar to the Booster Cookie.
To activate a cake's effects, they must be right clicked while placed down on the ⏣ Private Island. Cakes placed on others' Islands can grant their effects. Once placed down, they will never disappear, work as Furniture (but have their own furniture limit), and can be used endlessly. The cakes may be eaten again before the 48 hours are up to refresh them and reset the countdown to 48 hours. A maximum of 14 can be placed on an island.
If the player has more than 8 hours of playtime, a Century Cake will have a 0.1% chance to drop a Cake Soul of its color when eaten.
The Streamer's Century Cake giving +1♣ Pet Luck is a reference to streamers feeling luckier than regular players. It could also be a reference to 7Refraction, who has a popular clip of him having horrible luck with maxed ♣ Pet Luck.
The Century Cake of Hype grants +5☘ Mining Fortune. It possibly refers to the Hype Meter and the Hype Diamonds in the Prototype Lobby, and may be a memorial of that Hypixel Skyblock has received enough Hype Diamonds to be released.
The remaining cakes seem to have no specific references to Hypixel, but are instead a reference to their color.
The Not-a-lie Century Cake is the only Century Cake that seems to refer to something outside Hypixel, specifically the series Portal, where the player character Chell is promised Cake, but as hinted by secrets in the game, the promise, or the cake, is a lie, not to be confused with The Cake obtained from the Royal Resident.
When removing the aPunch Century Cake, the chat message displays the name as an APunch Century Cake, with the A capitalized, whereas placing it, and the item name, refers to it as an aPunch Century Cake.
On Alpha Hypixel Network, before the Year 300 event, players could obtain cakes on year 299, while on every other occasion they were obtained on a year divisible by 100.
Simon or the 
Incredible Raffle Box during the entire SkyBlock Year 100, Year 200, Year 300, and Year 400. They are placeable items which give small Stat bonuses when consumed.
Crab Hat of Celebration, one random type of Century Cake is given out to each player. Also like the 
Booster Cookie.
Cake Soul of its color when eaten.
Lime Dye
Rose Red
Dandelion Yellow
Light Blue Dye
Lapis Lazuli
Ink Sac
Orange Dye
Purple Dye
Cocoa Beans
Bone Meal
Cyan Dye
Cactus Green
Gray Dye
Magenta Dye
Light Gray Dye
Crab-Colored Century Cake is a reference to the 
Added Chocolate Century Cake.
Added Cloudy Century Cake..
Added Century Cake of Hype.
Added Not-a-lie Century Cake.
Added Undead ༕ Century Cake.
Added ekaC yrutneC tfiR.
Added Corpse-Flavored Century Cake.
