Break the chicken breast chunks into small pieces in a large bowl. Mix in the celery, onion, and grapes. Season with dill and salt and pepper.
Whisk together the mayonnaise, sugar, and vinegar in a small bowl until the sugar dissolves. Pour the dressing over the chicken mixture, and mix until combined. Cover and refrigerate for 30 minutes before spreading the salad onto the croissants to make sandwiches.