When John Kemeny was chair of the Mathematics Department at

When John Kemeny was chair of the Mathematics Department at Dartmouth College he received an average of ten letters each day. On a certain weekday he received no mail and wondered if it was a holiday. To decide this he computed the probability that in ten years he would have at least 1 day without any mail. He assumed that the number of letters he received on a given day has a Poisson distribution. What probability did he find? Hint: