He was so entranced by the numerical symmetry of his son Zak's birth on the 17th of January '71 and his weight of seven pounds, seven ounces that he insisted his son's middle name ought to be a number. (Although it remains a mystery why, with quite so many sevens in the equation, Zak's middle name is 9.
-- Alyn Shipton in Nilsson: The Life of a Singer-Songwriter
Harry had no other choice than "Nine" as the middle name for his first son, Zak Nilsson.
Adding a nine to the digits in Zak's birthday creates symmetry.
If you list the date of Zak's birth with the month first, it is:
Add a nine after the first one and you get the year repeated:
1 9 7 1 - 1 9 7 1
Even better, if you list the date of Zak's birth with the day first, it is:
Add a nine after the first seven and you get a palindrome:
1 7 9 1 - 1 9 7 1
So, Zak needed a nine. Ergo, Zak Nine Nilsson!