Grapefruit, which is in season September through April, can be extremely bitter, even when it's at its juiciest. Some people douse grapefruit in sugar or an artificial sweetener to cut through the tart acidity; however, a sprinkle of salt, although it seems strange, is an effective way to neutralize its bitterness.
Try it out at home by cutting the grapefruit into bite-size wedges, and then sprinkle it with salt. Keep your salting hand at about a two-foot distance, so the salt disperses evenly. As you eat the grapefruit, you'll be amazed by how much sweeter it tastes.