9 is the largest single digit integer
9 is the square of a prime and is the second lowest such square

"Casting out nines" is a quick way of testing the calculations of sums, differences, products, and quotients of integers. It was known as long ago as the 12th Century. It is also the basis of a lot of "magic" tricks and puzzles. There used to be some based on the rotary phone dial too. Essentially, They are modulo 9 math.

A number is divisible by 9 if its digital root is 9. A number's digital root is the sum of its digits. If such sum is greater than one digit, then those digits are also summed, etc. Thus the digital root of 18 is 9 (8+1), and the digital root of 456 is 6 (4+5+6 = 15 and 1+5 = 6)

Now the fun:
1/9 = 0.11 followed by an infinite string of 1s
2/9 = 2 x 0.11 to infinity = 0.22 followed by an infinite string of 2s
3/9, of course, is 0.33 fol;lowed by an infinite string of 3s, etc.
This brings us to
9/9 = 0.99 followed by an infinite string of 9s, which is also 1. Back in the stoneage, a test of calculators and computers was to perform the operation (1/9)x9 and see if it returned 1 or 0.9999999 etc.

Conversely:
123456789 x 9 = 111,111,111
123456789 x 18 = 222,222,222 (because 18 = 2 x 9)
etc up to
123456789 x 72 = 888,888,888 (9 x 8 ) and
123456789 x 81 = 999,999,999 (9 x 9)

9 is the number of lives a domestic cat has
9 is the Jersey number worn by the inimitable Ted Williams.
9 is Fluorine.
9 is Organic: Five-digit produce PLU codes that begin with 9 are organic.
A 9 sided polygon is a nonagon, aka an enneagon
There are 9 justices on the US Supreme Court

TITLE 9 of the U.S. Code is Arbitration

9 BCE was the Year of the Consulship of Drusus and Crispinus.
Nero Claudius Drusus died and Emperor Ping of Han was born.

9 CE was the Year of the Consulship of Sabinus and Camerinus
The Rhine river was established as the boundary between the Latin- and German-speaking worlds, following the defeat of the Roman army, under the command of Varus, at the Battle of the Teutoburg Forest. (Varus again)
Italy adopted the Lex Papia Poppaea outlawing celibacy and childless relationships, unlike later under the popes, who favored celibacy, though still abhorring childless relationships.
In China, Wang Mang founded the Xin dynasty, which lasted 16 years

On this day in:
1386 -- England and Portugal signed the Treaty of Windsor,the oldest diplomatic alliance till in force.
1911 -- The works of Gabriele D'Annunzio were placed in the Index of Forbidden Books by the Vatican.
1926 -- Admiral Richard E. Byrd and Floyd Bennett claimed to have flown over the North Pole
1945 -- Ratification of the German unconditional surrender of May 8 in Rheims, France,
1950 -- Robert Schuman presented his proposal on the creation of an organized Europe,
1955 -- West Germany joined NATO.
1958 -- The film Vertigo has world premiere in San Francisco.
1960 -- The FDA announced it would approve birth control as an additional indication for Searle's Enovid which made Enovid the world's first approved oral contraceptive pill.
1974 -- The United States House Committee on the Judiciary opened formal and public impeachment hearings against Nixon.

Born this day in:
1800 -- John Brown, a political activist
1882 -- Henry J. Kaiser, a shipbuilder and businessman, founded Kaiser Shipyards
1883 -- José Ortega y Gasset, a philosopher, author, and critic
1921 -- Daniel Berrigan, a priest, poet, and activist
1935 -- Nokie Edwards, a guitarist and actor (The Ventures)
1937 -- Dave Prater, a singer (Sam & Dave)
1942 -- John Ashcroft, a lawyer,politician, Attorney General, and twisted religio-fascist
1949 -- Billy Joel, a singer, songwriter and pianoman
1962 -- Dave Gahan, English singer-songwriter (Depeche Mode)

Died this day in:
1850 -- Joseph Louis Gay-Lussac, a chemist and physicist
1957 -- Enzio Pinza, an actor and singer
1976 -- Ulrike Meinhof, a militant, activist and co-founded the Red Army Faction
1979 -- Eddie Jefferson, a singer and lyricist
1986 -- Tenzing Norgay, a Nepalese mountaineer, Sir Edmund Hillary's enabler
2010 -- Lena Horne, a singer, actress, and activist

Holidays, Holy Days, Festivals, Feast Days and such:
It really is the anniversary of Dianetics, which is celebrated by Tom Cruise and others of his ilk.
It is also Europe Day, commemorating the Schuman Declaration, in the European Union.

So, for music we gots --
Numbah 9
Nokie Edwards
Dave Prater
Billy Joel
Dave Gahan
Enzio Pinza
Eddie Jefferson
Lena Horne

Nokie Edwards

Dave Prater

Billy Joel

Dave Gahan

Enzio Pinza

Eddie Jefferson

Lena Horne