So, suppose we would like to know the likelihood of receiving exactly 1020 requests in a 10 minute time interval. This means we need
\begin{equation*}
P(X = 1020) = f(1020) = \frac{1000^{1020}}{1020!} e^{-1000}
\end{equation*}
which might be totally impossible to compute directly using a regular calculator. However, many graphing calculators have a built-in function where f(x) = poissonpdf(mu,x) and F(x) = poissoncdf(mu,x). To answer our question,
\begin{equation*}
f(1020) = \text{poissonpdf(1000,1020)} \approx 0.01024.
\end{equation*}
On the other hand, suppose the question is to ask whether 1020 or fewer requests wil be made in the 10 minute interval. If so, then
\begin{equation*}
F(1020) = \text{poissoncdf(1000,1020)} \approx 0.74258.
\end{equation*}