The holidays are a time for relaxing, helping the less fortunate, showering family and friends with love and attention—and, sometimes, for smiling and nodding through unsolicited stock tips from an overbearing relative who has been sampling the eggnog.

But good advice can make careers and forever change lives for the better. So The Wall Street Journal asked an array of prominent people who manage, invest, study and write about money to share the single best piece of financial advice they ever received—or gave.

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