You'll also want people to RSVP so you have a head count for the caterer. If, on the other hand, your rehearsal dinner will be fairly low-key or small (think: a party at a restaurant or an intimate gathering at your future in-laws' home) then you don't need to be as official with your invitations.
The exact nutritional value of an orange depends on its variety. According to the U.S. Department of Agriculture, a standard-sized navel orange contains 69 calories, 17.6 grams of carbohydrates and 11.9 grams of sugar.
There are 45 calories in 1 small [4.6 oz as purchased, 2.4 dia] (3.4 oz) of Oranges, average all varieties, raw. You'd need to walk 12 minutes to burn 45 calories. Visit CalorieKing to see calorie count and nutrient data for all portion sizes.