Best way to determine Lambda to calculate Poisson probabilit

[Aetius]

What is the best procedure to estimate Lambda to calculate the Poisson probability? Say I want to calculate P(X ≥1) of an accident occurring next day. For this I would calculate the average of daily accidents and divide it by 10. The question is, should I take the previous 10 days? Or calculate λ averaging i.e. 10 day periods for the last 200 days and divide them by 20? What would be the best?
Thank you much for any constructive comment.