From "The Greek Myths" by Robert Graves (Penguin 1960):
"Adonis (Phoenician: adon 'lord') is a Greek version of the Syrian
demi-god Tammuz, the spirit of annual vegetation... the Adonia, a
mourning festival in honour of Tammuz, was held at Byblus every
spring."
See also Ezekiel 8:14: mourning for Tammuz reached Jerusalem also, and
was condemned as idolatry.