He instructed the White House to save a bird given to the president. His son had grown fond of the bird (and the president was a big animal lover).

MORE INFO:

John F. Kennedy then started a trend by publicly sparing a turkey given to the White House. He decided after receiving a bird on November 19, 1963, that it shouldn’t stay as dinner. The turkey was wearing a sign that said, “Good Eatin’ Mr. President.” JFK spared the bird just three days before he was assassinated in Dallas.

Lyndon Johnson, Richard Nixon, Gerald Ford, and Ronald Reagan were all photographed at turkey press conferences with their guests of honor. It’s not 100 percent known if any of the birds survived their White House tour—without being stuffed, dressed, and served on a platter. Reagan joked about pardoning a turkey during the days of the Iran-Contra affair, but the bird was already scheduled to live out its life at a zoo.

It was President George H.W. Bush who made the turkey pardon official when he took office in 1989. Since then, turkeys across the United States have rejoiced, at least one day a year, as the leaders have spared a lucky bird from the Thanksgiving table.

FROM: https://constitutioncenter.org/blog/the-real-story-behind-the-presidential-turkey-pardon