Plenty of Interweb users out there have mused, "What does the Leviticus spell do in Harry Potter?" Well, musers of the World Wide Web, there is no Leviticus spell in Harry Potter.
Thinking that there is, though, is a natural mistake. J.K. Rowling takes spells like "Wingardium leviosa" from Latin. Well, maybe not every word is real Latin as used by actual Romans, but close enough. Leviticus is from Latin, too—specifically (specificus?) from the Latin version of the title of the book in the Septuagint, the ancient Greek translation of the Torah.
In Greek the title is Leuitikon, which in Latin becomes—ta da!—Leviticus.
The Big Reveal
Okay, but why Leviticus? Because if the book were an episode of Friends, it would be translated "The One with the Levites."
Who are the Levites? They're the tribe of Israel set apart to maintain the Tabernacle. The details will become a lot clearer in the book of Numbers, but the gist of it is that the tribe of Levi is made up of three family clans. The descendants of Aaron are the priests, while members of the other two Levite clans do the Tabernacle's scut work: feed grapes to the priests, sell popcorn to tourists, clean the Portajohns—everything the priests are too stuck up to do.
The Levites who aren't priests don't show up much in Leviticus, but at least they get a book named after them. Take that, Naphtali and Issachar.
The Name Game
But remember, the Septuagint isn't the original version of the Torah. Instead, it could be described as the ancient Greek ancestor of the King James Bible. It's a compilation of Greek translations from the original Hebrew that, according to legend, was produced by 70 scholars in the 3rd century BCE.
In Hebrew, there are a couple different titles for this book, neither of which is Leviticus. One title for the book in Hebrew is Torat Kohanim, or "Instructions for Priests." This title is especially popular among people named Cohen, since it keeps the rest of the Levite tribe from bogarting their fame.
But the best known and most commonly used title in Hebrew is taken straight from the first two words of the first verse: Va Yikra, or "And he called." Religious teachers have for centuries read Va Yikra as a sign of God's care for his people. After all, if he had started the book with something like "The liver lobe" or "Unclean semen," the title would have been impossible to say out loud without someone giggling.