It's best to prepare this the day or night before you want to serve it, but not more than that. This also makes it the perfect dessert for summer entertaining: It's cool, light, refreshing, and impressive.
This was so yummy, very flavorful! Also very fast to prepare, squeezing the grapefruits was the most time consuming thing. I would consider just buying grapefruit juice in the future. Tip: I lined the pan with saran wrap so when it was set, I just tipped it upside down onto a cutting board, and pulled a bit on the wrap to get it out. This made it much easier to cut in those neat little cubes.