Probabilities involving random arrivals

Input the number of random arrivals ( such as births ) and the time (in days ) during which the observations were recorded. You will get a probability distribution of the number of arrivals you can expect per day. Then, enter a number of concern, and get the probability that you you observe fewer than or equal to that number.
This is a study involving the Poisson Distribution. Do some research online to find the formula for it. What is the mean for this distribution? What is the standard deviation?