If there's one thing I love to do, it's match the food to the occasion. While you could certainly make a berry flag cake for Memorial Day, that's a bit literal for my taste. A more subtle way to inject some holiday color into your menu would be pureeing the berries and serving them as a cold soup.
Blueberry Soup is a unique crowd pleaser...here's an easy recipe. It's great as a sweet appetizer, a palette cleanser, or dessert. If you want to play with color a bit more, garnish with sliced strawberries.