Czenglish: In these days I am very busy.
English: I think ‘these days I am very busy’ is better for what you want to say.
The difference is small but ‘in these days‘ is a little more formal and carries a little more weight. For example you might say:
“In these days of political unrest and economic uncertainty it is difficult to plan for the future.”
Meanwhile ‘these days‘ is generally better when you are talking about yourself or situations that aren’t especially serious. Realistically, how busy you happen to be will always fall into this category.
So, “These days I am very busy.“