With his band of Death Eaters, Voldemort’s first reign of terror begins. His ascent to power is marked by strange disappearances.
This comes from Dumbledore's statement in PS1 that "we've had precious little to celebrate for eleven years." At that time it is November, 1981. Though we don't know exactly how Voldemort's ascent to power happened, it is probably safe to assume that it somehow took a big leap in 1970.