Of course, the only hit song by the Zombies came to life after the band was dead. The band released "Time Of the Season" in 1968, then without delay broke up, only for their single to break out in 1969. The Zombies' undead classic continues to stalk the world!
This song employs a trick I love, alternating between the major and minor of the same chord, requiring just a half-step bump of a single note (the third) to become a new chord and noticeably alter the key. The majority of the song is in E-minor, but during that long refrain line ("It's the time of the season for loving"), the resolving chord becomes an E-major, and the shift of the third up a half step to make the major chord is the melodic hook of the song. It's a cheap thrill, but it gets employed over and over in songs throughout generations in new and seductive ways (Paul McCartney used it in "The Fool On the Hill" around the same time). And of course, it was a staple in classical harmony long before - no artist is above a cheap technique that really works.
It's a superb melodic song, employing a cool mode to create a collection of fragments that combine into long phrases. There are arpeggiations that get call-and-response echoes. There are downward scales that find pretty low notes at creative intervals. There are climbing intervals on which the verses peak. The lead up to the refrain has another great set of call-and-response ideas with more of a classic rock n' roll feel.
The radiant harmonies of the refrain are the the reason we listen. By contrast, the background vocal "aah"s are basically subliminal, but they're also imperative additions, setting this thing apart.
The lyrics are pretty standard late-'60s cliches. You can argue that this song is more of a calculated fabrication, something an ad company would come up with to approximate the more impactful "counterculture" statements of bands like the Byrds. You may be right, and the band obviously lacked a strong sense of mission, as its dissolution proved. And what delegitimized the '60s counterculture more than ad companies mimicking the rhetoric to sell cars and antacids?
But it is the perfect zombie song, letting a band rise after its death, a catchy jingle getting more respect as its era cooled in the ground. It was also the banner song in the great film Awakenings, where people immobilized since the '60s are reanimated and walk the Earth again.