Line an 8x8 or 9x9 inch square baking pan with aluminum foil, leaving an overhang around the edges.
In a large bowl whisk together the pudding mix & milk.
Gently fold in the lemon zest and 2 cups of the whipped topping.
Spread 2 tablespoons of the lemon filling onto the bottom of the pan.
Place a layer of graham crackers over top. You may need to break or trim some of the crackers.
Spread 1/2 of the lemon filling over top.
Place a layer of graham crackers over the lemon filling. Then add another layer of graham crackers.
Spread 1 cup of whipped topping over top.
Place in the freezer for 4 hours to set.
When ready to serve, lift the cake out using the aluminum foil overhang and cut into pieces. If it's been in the freezer for over 4 hours, I recommend letting it soften in the fridge for 30 minutes before enjoying.