Northeast of Kathmandu, the Three Passes loop is a popular trekking circuit totally 80 miles with 49,700 feet of elevation gain. The Three Passes trek offers views of the large peaks within the Eastern Himalayas of Nepal. The trek typically takes about 20 days and is best in April or October. Even trekking in the best months can offer many cloudy days with limited visibility. Since this is a popular route to the Everest base camp, you'll want to book all your lodging in advance.
Most trekkers arrive into Nepal via Kathmandu and stay a night or two in the Thamel neighborhood which is know for its trekking supplies. By the way, bring adequate cash (at least 1,000NPR/day) for the entire trip since ATMs are rare and travelers checks may not be widely accepted. This is not an inexpensive trek as many seems to falsely believe. It is not uncommon for trekkers to run out of money.
You will want to fly into Kathmandu Tribhuvan Internatioal airport. It's best to stay somewhere that has an airport shuttle. Most of the trekkers will fly on to Lukla to start the route.
Overall, Three Passes Circuit is the 2nd most popular hiking trail of all 3 hikes in Nepal.