Well, from what I can tell (and someone else may correct me on this), the first one is more of "I'm busy on Sunday" whereas the second is "It's a busy Sunday." The に (to something) indicates something directly related to you, whereas the は (is something) indicates a statement you are simply making.
Hope that helped!
November 12, 2013
Hi Elena さん、
Understanding the subtle difference between ”に” and "は" can be difficult.
I will try to make it easy for you to understand.
I am busy on Sunday.
This answer could be used to answer someone who asked you what particular days are busy for you.
Teacher: Elena, when is work the busiest for you?
Elena: I am busy on Fridays and Sundays.
This sentence can be used when someone uses Sunday as the main subject.
Friend: Elena, how about Sunday (to go out, see a movie, etc.)
Elena: Um.. I am a bit busy on Sunday. (lit. Sunday is a bit busy [for me])
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