If Your intended to do Everest base camp trekking we will recommend you out 3 months in advance. For Everest base camp trekking we have to reserve air ticket to Lukla. On Sept to Dec the air ticket sold out first cum first basis. For your kind information the Lukla flight are few airlines are operation in Nepal so that we will recommend you to reservation your trek as far in advance as possible as soon as you know the dates of your international flights.