The effect of the summer monsoon season in the Everest region is very pronounced, with heavy rainfall effectively closing down the region. Trails can become extremely difficult, with slippery mud and rocks, and most lodges close. In addition, the trails can also see outbreaks of leaches, therefore trekking is not recommended during this season.
Spring and Autumn then see much drier and brighter days, making March to May and September to November great times to go trekking. Rainfall is still not an issue in winter, however you will have to face severe snowfall and some high passes become unsafe to cross. Winter also brings very cold temperatures, particularly at night, so although this season sees quiet trails you will need top-quality sleeping bags and clothing.