The mid Dec is quite late time to do the Everest basecamp trek, there were chilly freezing nights, but the up side was there were not many tourists there. 

Access: through Lukla airport (considered on of the most dangerous airports in the Workd due to its shortness). But the pilots are professionals, so it is basically safe. 

Flights to Lukla are canceled in a bad weather. It might be canceled just moments before. 

You need a permit for the area. 

Permits and flight tickets acquired by namastetrek.com.np (Jangbu Sherpa). 

Main season for the trek is considered Mar-May & Sep-Dec. May is quite hot.