Cook the macaroni according to package directions until perfectly al dente. Undercooking slightly is key as it will absorb dressing later.

Drain the cooked macaroni immediately and rinse thoroughly with cold water to stop the cooking process and remove excess starch. Drain very well.

While the pasta cools, prepare the dressing: In a large bowl, whisk together the mayonnaise, white vinegar, milk, sugar, salt, black pepper, and onion powder until smooth and fully incorporated.

Add the rinsed, drained macaroni, finely diced celery, and minced sweet onion to the bowl with the dressing. Mix gently until everything is evenly coated.

Cover the bowl tightly and refrigerate for at least 4 hours, but preferably overnight, allowing the flavors to meld and the pasta to fully absorb the dressing.

Before serving, taste and adjust seasoning if necessary (you may need a pinch more salt or sugar). Serve chilled as a classic side dish.