Why do people "knock on wood"?

Search: knock on wood origin

Why: Friday night, Sam told Mike to knock on wood at the bar. He tapped on a tree. Later, Melia knocked on our kitchen cabinets.

Answer: First, some history!

• 16th c. - Latin phrase absit omen, "far be that omen from us," cited in John Heywood Jablowme's collection of proverbs
• c. 1850 - English scholarly journal Notes and Queries publishes:

There probably is some old English expression for averting evil, but it does not come to mind; "I touch wood," "Bar omen," "Bar ill-luck," seem clumsy.

(in the UK, people say "touch wood")

• 1905 - The Syracuse Herald publishes:

Neglecting to knock on wood may have been responsible for the weather's unseemly behaviour today.

The actual origin is unknown, but a popular theory looks to pre-Christian tree spirits in Ireland. They live/d in sacred trees like the ash, hawthorn, holly, and oak, and people "knocked on wood" either for good luck or to thank them for their good luck. Or: the sound you made knocking would prevent the Devil from hearing an unwise comment you just made. Or: it had something to do with the wood of the Christian cross (but probably not)

Also:

It wasn’t always wood that was lucky: in older days, iron was also thought to have magical properties, and to touch iron was an equivalent preventative against ill-fortune.

Source: Phrases.org.uk, World Wide Words

The More You Know:

Shall I compare thee to a summer's day?
Thou art more lovely and more temperate:
Rough winds do shake the darling buds of May,
And summer's lease hath all too short a date:

(etc.)

The "darling buds of May" are not from the month, but from the May Tree, aka the Common Hawthorn.

In Ireland a hawthorn standing alone in open ground is known as a fairy tree and there is a strong superstition that to cut one is unlucky. Even in recent years roads in Ireland have been rerouted to avoid uprooting hawthorns. It is also considered unlucky and an omen of death to cut the blooms and bring them into a house. This may well have come about from the unpleasant aroma, which is like decaying flesh.

Grimm.