What is the difference between 金曜日にテストがある and 金曜日はテストがある?
The emphasis of the sentence. Basically “We have a test on Friday” versus “On Friday, we have a test”.
Belthazar already answered properly. Maybe I can complement and present the way I interpreted this kind of sentence when I learned them.
金曜日は the は particle establishes topic of what you’re saying, what you’re talking about.
金曜日に the に particle establishes the when the action takes place.
When establishing topic, I kinda interpret it that way: “About Friday, there’s a test”. You’re talking about Friday, and it just so happened there’s a test. As Belthazar pointed out, emphasis is on the day itself, not the test.
In the second sentence, you’re really trying to say that there’s a test on Friday.