December 22nd, 1963 was a Sunday, so one can make out of that whatever one wants. I say that means, the paper saved up an important piece, anything written by an ex-president for the Sunday papers.
The letter was dated December 21st. It was printed December 22.
The paper didn't "save" that letter. They printed it immediately.
Truman wrote it less than a month after JFK was assassinated. If he wanted to avoid making a correlation between the assassination of JFK and the CIA, waiting 24 hours before releasing it would have served that purpose. Waiting until after New Years would have separated the event from his statement even more so. He chose not to do either. He chose to tie his statement about the renegade CIA and the assassination together by balancing tact, the immediate urgency of the message, and symbolic calendar date with the release of his message.