Luck in odd numbers. A major chord consists of a fundamental or tonic, its major third, and its just fifth. According to the Pythagorean-system, “all nature is a harmony,” man is a full chord; and all beyond is Deity, so that nine represents deity. As the odd numbers are the fundamental notes of nature, the last being deity, it will be easy to see how they came to be considered the great or lucky numbers. In China, odd numbers belong to heaven, and v.v. (See Diapason, Number.)
“Good luck lies in odd numbers ... They say, there is divinity in odd
numbers, either in nativity, chance, or death.” —
No doubt the odd numbers 1, 3, 5, 7, 9, play a far more important part than the even numbers. One is Deity, three the Trinity, five the chief division (see Five), seven is the sacred number, and nine is three times three, the great climacteric.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Odd Numbers from Fact Monster: