In late 1960s Birmingham, the heart of England’s Rust Belt, singer John “Ozzy” Osbourne, bassist Terrence “Geezer” Butler, guitarist Toni Iommi, and drummer Bill Ward made up a blues band called Earth. They struggled to make it on the club circuit, and then they discovered that there was already another blues band called Earth.
As they were looking out the window from their rehearsal room at the movie theater across the street, they saw people queue up for a horror movie featuring Boris Karloff, “Black Sabbath”. The full movie can be seen here in 1080p on YouTube.
Then the band had an epiphany. “If people pay money to see scary movies, maybe they’ll pay to hear scary music.” “Geezer” and Toni started bouncing around with the tritone-based theme of “Mars” from Holst’s “The Planets”, and before they knew it, they had put together a song with horror-themed lyrics by Geezer, sung by Ozzy with precisely the right “vibe”. They tried the composition out at their next gig — and when the crowd went mental and made them play it three more times, they knew they’d struck gold. They changed their name to Black Sabbath, and the rest is history.
Here is a live performance from 1970:
And a version from their farewell tour (a half-step down relative to the original)
There is something of an urban legend in music (much enhanced in the retelling by metal musicians trying to capitalize on how “evil” one of their favorite intervals sounds) that the medieval church prohibited the tritone as the “diabolus in musica” (Latin: devil in music). The truth is a bit more complex, as Wikipedia explains (caveat lector, as always with WP):
This interval was frequently avoided in medieval ecclesiastical singing because of its dissonant quality. The first explicit prohibition of it seems to occur with the development of Guido of Arezzo‘s hexachordal system, who suggested that rather than make B♭ a diatonic note, the hexachord be moved and based on C to avoid the F–B tritone altogether.
The name diabolus in musica (Latin for ‘the Devil in music’) has been applied to the interval from at least the early 18th century, or the late Middle Ages, though its use is not restricted to the tritone, being that the original found example of the term “diabolus en musica” is “Mi Contra Fa est diabolus en musica” (Mi against Fa is the devil in music). Andreas Werckmeister cites this term in 1702 as being used by “the old authorities” for both the tritone and for the clash between chromatically related tones such as F♮ and F♯, and five years later likewise calls “diabolus in musica” the opposition of “square” and “round” B (B♮ and B♭, respectively) because these notes represent the juxtaposition of “mi contra fa”.Johann Joseph Fux cites the phrase in his seminal 1725 work Gradus ad Parnassum, […] there are no known citations of this term from the Middle Ages, as is commonly asserted. […]
That original symbolic association with the devil and its avoidance led to Western cultural convention seeing the tritone as suggesting “evil” in music. However, stories that singers were excommunicated or otherwise punished by the Church for invoking this interval are likely fanciful. At any rate, avoidance of the interval for musical reasons has a long history, […] Later, with the rise of the Baroque and Classical music era, composers accepted the tritone, but used it in a specific, controlled way—notably through the principle of the tension-release mechanism of the tonal system.
Adam Neely also weighs in:
“Black Sabbath” of course wasn’t the first widely heard song to feature a riff based on a tritone. (As a passing note, it was already commonly heard in blues music.) Consider the intro of Jimi Hendrix’s “Purple Haze”:
ADDENDUM: Most Americans have no idea that Halloween isn’t a traditional thing in continental Europe or in Israel — or that indeed, especially in the Lutheran parts of Germany, October 31 is a different holiday entirely named Reformation Day (the commemoration of Martin Luther sticking his 95 Theses on the Wittenberg church door on October 31, 1517 — the conventional starting date of the Reformation).
Celtic pagan culture had a holiday named Samhain (freely “summer’s end”), the end of the harvest season and the beginning of winter. This was one of the great four [trimester] feasts. together with Beltain (approx. May 1, beginning of summer), Lugnasadh (approx. August 1, beginning of harvest), and Imbolg (approx. February 1, a purification festival also seen in Ancient Rome). Celebrations for the eve of Samhain, Oíche Shamhna in Gaelic, were apparently marked by customs not unlike Halloween (contraction of All Hallows’ Eve).
All Hallows Day itself, in the early Middle Ages, had been observed on various dates in various places, until in the 9th Century, Pope Gregory IV set November 1 this day aside for the commemoration of all saints (All Saints’ Day, All Hallows Day,… in Dutch and German, Allerheiligen; French: Toussaint). (November 1 is a public holiday in many European countries to this day.)
After the Great Schism, churches of the Eastern Communion (Greek Orthodox, Russian Orthodox,…) moved this observance to the first Sunday after Pentecost. November 2, All Souls Day, was meant for commemorating deceased relatives (fulfilling a similar function as the Yizkor service in synagogues): in some countries, notably Belgium, the two holidays kind-of merged into one as November 1 is a public holiday and November 2 a workday.