Yes, while trekking in Nepal; it is wise to allow some 2-4 spare days just in case of flight delays or bad weather, landslide and an avalanche may alter your itinerary. Also you may need additional buffer day to acclimatize your body while trekking up to Base Camp of the mountains as this is supposed necessary due to safety concerns. In addition, you will need transportation days from and to your home country.
An If you are trekking in restricted areas such as Upper Mustang, Kanchenjunga, Nar Phu Valley, Manaslu or Tsum Valley with it; usually we need one working day to get restricted area trekking permits. During weekends and public holidays; government offices remain closed. We can only issue your normal trekking permits in your absence or so called in advance. Restricted areas trekking permits only can be obtained in the presence of your original passport so you will need extra day in Kathmandu to get the permits.