にｖｓは　with days of the week

What is the difference? 日曜日にいそがしです。 日曜日はいそがしです。

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!

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. A: 日曜日に忙しいです。 I am busy on Sunday. This answer could be used to answer someone who asked you what particular days are busy for you. Teacher: エレナさん、仕事はいつが一番忙しいですか？ Elena: 金と日曜日に忙しいです。 Teacher: Elena, when is work the busiest for you? Elena: I am busy on Fridays and Sundays. 日曜日は忙しいです。 This sentence can be used when someone uses Sunday as the main subject. Friend: エリナさん、日曜日はどうですか？ Elena: あのう、日曜日はちょっと忙しいんです。 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])