3sleeves of graham crackers or about 24 graham crackers
Place a large mixing bowl and a stand mixer whisk or electric mixer beaters in the refrigerator for about 15 minutes to chill.
While the utensils are chilling, wash the strawberries and drain them on a paper towel. Remove the leaves and the core of the strawberries and slice them thinly. Set aside.
Wash the blueberries and drain on paper towels. Use another paper towel to dry them off. Set aside.
Add the cream, powdered sugar, vanilla, and salt to the chilled bowl. Beat on low for 3-4 minutes, then switch to high speed. Beat for a few more minutes or until the cream is whipped and holds stiff peaks.
Divide the whipped cream into 3 equal parts.
Place a spoonful of the whipped cream in the bottom of a 9x13 baking dish and spread out evenly. This holds the graham crackers in place.
Add a layer of graham crackers. You may have to break some of the crackers to evenly cover the bottom.
Use an off-set spatula to spread one-third of the whipped cream evenly over the graham crackers. Top the whipped cream with all but about 50 of the blueberries.
Repeat with another layer of graham crackers and another layer of whipped cream. Top this layer of whipped cream with sliced strawberries but make sure to reserve some for the top of the cake.
Add a final layer of graham crackers and the rest of the whipped cream. Spread the whipped cream out evenly.
In the upper left-hand corner, add five rows of blueberries with 8 berries in each row to form the stars. Add five rows of sliced strawberries overlapping them to form the stripes.
Cover with plastic wrap and refrigerate for at least 2-hours or overnight. To test to see if the cake is ready, insert a knife in the center. It should go in easily and should come out with soft crumbs.