You need to arrive no later than the morning of the first day of the tour. The first day is always organizational; on this day you will be able to purchase missing clothes (if necessary), participate in an organizational meeting and walk around the city. You can fly to Kathmandu the day before the start of the tour. The earlier you arrive, the more time you will have to rest after the flight, since on the second day of the tour early in the morning we will already have a departure to the protected area.
On the last day of the tour, we will return to Kathmandu around 16:00, then you can either fly out that evening (you will need to arrive at the airport 2 hours before departure and be sure to allow 40 minutes to get from the city to the airport).
Or you can stay an extra day or two in Kathmandu, as per your preference.