In a medium bowl combine the sour cream, milk, lemon zest and juice, Dijon mustard, garlic, sugar, and salt and pepper. Mix well to blend and loosen the sour cream. Stir in the minced dill. Let sit at room temperature for 20 minutes.
The sauce will last for a few days in the refrigerator, though it might start to separate a bit as it sits. Just give it a good stir before serving.
Notes
If you are watching your fat and calories, you can make a modified version of this sauce which will still be creamy and flavorful but lighter. Instead of full-fat sour cream, use reduced-fat, or use plain Greek yogurt. Note that Greek yogurt comes in different fat-level versions, from no fat to 1% to 2% and all the way up to 10%.