In a large bowl, mix macaroni (may have to wet it with cool water while its in the strainer after it cools to "unstick" it) , 3/4 cup of mayo, celery, onion, salt and pepper. Reserve 1/4 cup of mayo to mix in right before serving.
Refrigerate at least 2 hours before serving. Mix in remaining 1/4 cup of mayo right before serving. Will keep in fridge for 5 days.