Add the watermelon, strawberries, banana, almond milk, and date (if using) to a high-speed blender. Blend until very smooth.
Pour into a glass and serve right away for the best texture.
I like to add the date for a little extra-sweetness, but you can also add a squeeze of honey or maple syrup, as an alternative. If you want a slight tangy flavor, add a squeeze of lemon or lime juice, to taste.