The North is the coolest region of Vietnam where its seasons are clearly divided into four seasons: spring, summer, fall and winter. Best time to visit the North: from October to April.
Amongst which, rainy season occur from summer to autumn while dry season falls between the other two. Winter in Northern Vietnam usually bring extremely cold but dry weather, especially in the Northwest altitudes like Spa, Dien Bien or Lao Cai where snow falling occasionally in sub-zero condition. Hanoi and its surrounding province like Ninh Binh, Hai Phong or Quang Ninh has a milder climate in winter but much hotter and wetter with high humidity (80% on average) in rainy season (May-October) should worry things.
Best time to visit the North: from October to April because at this time, this area of Vietnam having its driest days and most favored temperature. This also includes your opportunity to enjoy the most festive time of a year in Vietnam when Tet Holiday takes place.