Hi, I am going to visit China from February 6 to 20, will it be too busy on those dates?

At that time, the public holiday has finished. There won't be many tourists in these cities. The 3 cities all have good destinations to enjoy the Lantern Festival.
In Beijing, you can attend temple fair in Jingshan Park and Fragrant Hills Park and also enjoy colorful ice lanterns in Shichahai and Longqing Gorge.
There will also be lively temple fair in Yuyuan Garden and Guyi Garden in Shanghai.
In Xian, you can go to the City Wall, City God Temple, and Tang West Market to feel the festival.