Cook the elbow macaroni in salted boiling water according to package directions until al dente. Drain well, rinse under cold water to stop the cooking process, and set aside to cool completely.

In a large mixing bowl, combine the cooled macaroni with diced celery, red onion, parsley, and tomato, if using. Toss lightly to distribute the ingredients evenly.

In a separate smaller bowl, whisk together the mayonnaise, sour cream, cider vinegar, dry mustard, sugar, and salt until smooth and creamy. This dressing should be tangy yet balanced with a touch of sweetness.

Pour the prepared dressing over the macaroni mixture and gently fold until every piece is well coated. Taste and adjust with additional salt and freshly ground black pepper as desired.

Chill the salad for at least 30 minutes before serving to allow the flavors to meld and the dressing to slightly thicken. Serve cold as a refreshing side dish — perfect for picnics, barbecues, or casual family gatherings.