What it means:
one's youth or heyday
How Shakespeare used it:
In Antony and Cleopatra, Cleopatra recalls her relationship with Julius Caesar that occurred during, "My salad days, / When I was green in judgment...." (Act 1, Scene 5)
Originally, English speakers used salad days with Cleopatra's meaning: a time of youthful inexperience or indiscretion. These days, however, it usually means "an early flourishing period" in other words, a heyday.
"I know there are some people who still think this show is good, or watchable.... Just because there are a few cool things left from the salad days doesn't mean Heroes isn't a mess." Darren Franich, EntertainmentWeekly.com, Nov. 10, 2009