which festival we really must see in Ningbo? and which other places we don't mist miss there.

As I know, there is no special local festival during February to April. But there are many must-sees in this city. Baoguo Temple, Tianfeng Pagoda, Tianyi Pavilion and Yandang Mountain are highly recommended.

About the weather, April is better because the climate will be warmer than February and March. It may 55 F to 75 F. You can bring light jackets, T-shirts and jeans.
Have a good journey!