The story
A man was warned by his teacher that disaster would come to his village on that day and told to take his family up a hill with chrysanthemum wine and dogwood. He did, and came back down to find the livestock dead. Climbing high on this day has been the custom since.
Too much yang was reckoned dangerous rather than lucky, so the day is about getting above it and airing the house out.
Because jiǔ, nine, sounds like the word for a long time, the doubled nine came to mean long life, and China made the date its day for the elderly in 1989. The pun did the work.