It's on Friday. 是回答某一個事件，可能是演講或是表演，會在禮拜五舉行，才會用這個回答。這句的不像前兩個回答，On Friday 可以當作是一個助動詞，形容購物的動作。
It is on Friday 這句話的主詞跟動詞 its (It is) 限訂了on Friday 只能形容一個事件，而不是動作。
參考以下的問題：When is Blink 182 concernt? It's on Friday. 注意看It 是代表演唱會，而不是動作。
2008-01-11 15:46:24 補充：
Action is something one's do or perform.
An event is an occurance happened at a specific time.
2008-01-11 15:46:33 補充：
Look at the question, 'What day does Amy go shopping?'. The question gives an impression, or to me at least, asking when this action take place. Therefore, you are looking for an adverb or an adjective describing an action.
2008-01-11 15:46:55 補充：
Now, going back to the answers
A: She goes shopping on Friday.
It is an complete sentence answering the date when the action shopping takes place.
2008-01-11 15:47:02 補充：
B: On Friday.
It is an abbreviated answer. Because 'On Friday' does not specified what it tries to descirbed, so it can go either way, and therefore acceptable.
2008-01-11 15:47:31 補充：
C: It is on Friday.
The subject and the verb of this sentence 'it is' denotes that 'It' as something, not action. Using an action 'running' as an example. Running is on Friday does sound awkward. It is acceptable to say 'Marathon', which is an event, is on Friday, but rarely vice versa.
2008-01-11 16:02:26 補充：
To John's question,
Well, grammar provides a set of standards for people to follow.
Let me answer your question first. You do bring a valid point, 'Amy go to the mall' is an event. Still, the question here still looks for when the action 'go' takes place, so A3 still not okay.
2008-01-11 16:05:29 補充：
The problem here is not the grammar, but the context. What kind of answer this question is looking for. Based on the context of your question, it is not looking for how long does 'goes' take, it is looking for when 'goes' take place.
2008-01-11 16:14:21 補充：
I do not know who set the grammar rules, but grammar rules are established to distinguish different tense, allowing people to have a more complete perception on the time, and the person of the articles they are reading, and therefore important.
2008-01-11 16:19:37 補充：
How long and where she live is not the scope of this question. 'Amy goes to the mall' only cares about when 'goes to the mall' takes place, specially the action 'goes'. If considering the entire phrase as an event, then the question needs to be rephrase.
2008-01-11 16:23:08 補充：
You don't ask 'what day does (event)?'. You say 'What day does (event) takes place?'.
I am not sure where you are going with computer programming. You mean polymorhpism, function overload, operator overloading or data abstraction?
自己不學無術, Why is the limit on English Character so small?