The days of one's youthful inexperience.

From Shakespeare's Anthony and Cleopatra, 1606:

CLEOPATRA:

My salad days,
When I was green in judgment: cold in blood,
To say as I said then! But, come, away;
Get me ink and paper:
He shall have every day a several greeting,
Or I'll unpeople Egypt.

'Salad days' is used these days to refer to the days of carefree innocence and pleasure of our youth. It has also been used to refer to the time of material affluence in our more mature years, when the pressures of life have begun to ease - something akin to 'the golden years'. Shakespeare meant the former, and the clue is in the colour. While he used green in other contexts to signify jealousy - 'the green-eyed monster' in Othello and, in Love's Labours Lost "Green indeed is the colour of lovers", it is used here to mean immature. The green of salad leaves, which are invariably short-lived, is an obvious allusion to youthfulness. Green is also used in other expressions to mean unready for use, for example, 'green (unripe) corn', 'green (unseasoned) timber and 'greenhorn' (an inexperienced recruit).

The phrase 'salad days' lay dormant for two hundred years or more but became used widely in the 19th century. For example, this citation from the Oregon newspaper The Morning Oregonian, June 1862:

"What fools men are in their salad days."

Salad Days was later used as the title of a highly successful is a musical, which premiered at the Bristol Old Vic in 1954. The music was written by Julian Slade and the lyrics by Dorothy Reynolds and Julian Slade. This was also the inspiration for the Monty Python spoof sketch Sam Peckinpah's Salad Days, in which the carefree young things featured in the musical were hacked to pieces in a typically gory Sam Peckinpah manner.