If you're looking for a reservation for a specific date/time, the quickest way is on the Schedule page. On the right-hand side you can switch between day/week/month views and on the left-hand side you can navigate to a different time period.
If you're looking for a reservation of a specific customer (by their first or last name, login, user ID, email address etc.) or if you know the reservation ID, you can quickly do this in Reservations > Search.