The word "七夕" is formed by combining the character "七" meaning "seven" with "夕" meaning "evening", directly denoting the seventh evening of the seventh lunar month, which corresponds to the annual Qixi Festival.
Word Definition - 七夕
qī xīdouble seven festival, evening of seventh of lunar seventh month; girls' festival; Chinese Valentine's day, when Cowherd and Weaving maid 牛郎織女|牛郎织女 are allowed their annual meeting