Bring a large pot of salted water to a rolling boil. Add the elbow macaroni and cook according to package directions until al dente, usually 7-9 minutes. Drain thoroughly in a colander and rinse with cold water to stop the cooking and cool the pasta. Drain again very well, shaking the colander to remove excess water. Transfer the cooled macaroni to a large mixing bowl.

While the macaroni is cooking, finely dice the celery, red onion, red bell pepper, and green bell pepper. Chop the hard-boiled eggs (if using) and fresh parsley. Add all prepared vegetables, eggs, and parsley to the bowl with the cooled macaroni.

In a separate medium bowl, whisk together the mayonnaise, yellow mustard, apple cider vinegar, granulated sugar, salt, black pepper, and cayenne pepper (if using) until smooth and well combined. Taste and adjust seasonings if necessary; you might want a little more sugar for sweetness or vinegar for tang.

Pour the creamy dressing over the macaroni and vegetable mixture. Stir gently but thoroughly with a spatula until all the macaroni and vegetables are evenly coated.

Cover the bowl tightly with plastic wrap and refrigerate for at least 2 hours, or preferably overnight, to allow the flavors to meld and the salad to chill completely.

Before serving, give the salad a good stir. If it seems too thick, you can add a tablespoon or two of milk or a little more mayonnaise to reach your desired consistency. Serve cold.