The letter says that Cousin George would arrive 'today'. This implies that the letter was written and mailed on the same day that it was supposed to be delivered. That could not happen, as mail was sorted by hand at that time. In 1957, automated mail sorting in the U.S. was just starting to be introduced. Same-day local delivery is available through the U.S Postal Service now (2022), but only for businesses, and, as of this writing, only in specific ZIP Codes. It appears that the service will be available nationwide by the end of the year.
When Tom moved the TV in front of Jerry's mouse hole in the wall he did not plug it in so it would not work when George tried to turn it on.