The title might be hard to understand so I will give an example instead:

Every player will receive 1 pot with pine seed for free to place it in their condo or casino. Let's name the item is "Xmas tree"

During the Xmas event (from 5th Dec to 5th Jan), everytimes you receive a CP from games (poker, blackjack, soliatire, slot, ...), doing job, you will receive an extra "xmas decoration item" (act as a currency, not an item) so you can use it to grow your xmas tree in the condo. This currency is unmarketable but can be bought using gold.

The "Xmas tree" will have infinity level, everytime it level up, it will give some free XP, chip, CP and a random item. The more level the tree, the more stuff it gives and provide more popular stats. But the higher the level, the more "Xmas decoration" it requires.
About the models, this Xmas tree will have 10 to 20 models for the first 10 to 20 levels. Then from level 21 to above, it will only use the latest model.

After the event duration, the player can still level their tree up and get free stuff, but the "xmas decoration" will stop dropping. Also the population effect will be stopped. This item then either can be exchanged for some CP/XP/Gold (if it has very high level) or the player just keep it as a memorial object.

This is just an example, we can make a cupid statue for Valentine event, Sand castle for summer event, Halloween stuff,....