This is a pretty good explanation:
Sammy Hagar’s “There’s Only One Way To Rock” And Its Relation To The Subject’s Schedule: A Critical Analysis
In his 1981 hit “There’s Only One Way to Rock,” Sammy Hagar outlines the very specific criteria in making rocking possible. However, Mr. Hagar never explicitly defines or describes this singularly hypothesized way of rock. He, does, however, state the exact, proper time to rock: 8:05 (as in the lyric, “8:05, time to rock.”) This is a surprisingly early, yet extremely specific time for rocking, which warrants a full audit of Mr. Hagar’s schedule and a rundown of obligations which should also answer the underlying question: Why is Sammy Hagar, a man so clearly and publicly dedicated to rocking, unable to rock until shortly, and specifically, just after 8 p.m?
But at 8:05, the responsibilities of the day have been completed, and Mr. Hagar’s soul, spirit, and physical being are sufficiently prepared to rock. Mr. Hagar then affirmatively states – to himself, but often out loud – that it’s “8:05, time to rock.”
At this time, Mr. Hagar rocks.