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.

Modern example:

"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