Just a theory. The last we see of Verin is when she is telling Egwene about the Black Ajah. Verin had to wait until the hour of her death to get around her oaths to the Dark Lord. Maybe, the letter she gave to Mat was the Dark friends Verin knew about in Caemlyn and she couldn't let Mat know until she was dead. Verin had a satchel of these sealed letters, and she had traveling to do. The five days would have given her time to deliver all of the letters. The fifth day would have also been after her death, and give her a way around her oaths.