So far, there have been six seasonal invasions, Seaonal: Spring Break , which was available in Version 0.86 and Version 2.06, Seasonal: Easter which was available in Version 0.87, Seasonal: Brain Freeze which was available in Version 0.89, Seasonal: Halloween which was available in Version 1.05, Seasonal: Valentine's Day which was available in Version 1.12 and Version 2.15 and Tree World in Version 1.15 and Version 2.11.
There was also the Christmas version of the Old McDonnell's Farm Invasion in Version 0.82 but that is not really classed as a Seasonal Invasion.
The first loot items are both natural decor and are avaiable in the market
The second ones are items which are not in the market
The legendary items are all mobile
The Invasions offer similar mechanics with the exception of Brain Freeze. They all have things that take your zombies out of the fight, but do not kill them. Seasonal: Spring Break has the crab, Seasonal: Easter has the red balloon and Seasonal Halloween has a giant Pumpkin blocking the way. Seasonal Halloween has similar mechanics to the Circus Invasion except for its unique Brain Freeze where an icecream blizzard shrinks all your zombies in size and turns them blue, lowering their defence. Seasonal: Valentines Day has a chocolate fountain that traps your Zombies and Tree world makes Zombies turn into Turtles. All Seasonal invasions are unlocked at level 8.