In a medium pot, bring salted water to a rolling boil over high heat. Add the shrimp and cook for about 1.5 minutes if thawed, or 3 minutes if frozen. Drain and chill in the refrigerator.

In a large mixing bowl, combine the celery, mayonnaise, sour cream, chives, dill, lemon juice, Dijon mustard, salt, and black pepper. Whisk until smooth and creamy.

Add the chilled shrimp and cucumber to the dressing mixture. Fold gently to coat without breaking the shrimp.

Cover the bowl and refrigerate for at least 30 minutes before serving.