英语提问It's weekend.意思是”今天是周末”写成“Today is weekend.应该是可以的,但口语中我经

英语提问
It's weekend.意思是”今天是周末”写成“Today is weekend.应该是可以的,但口语中我经常看到的是前一句.为什么?

英语中,习惯用 it 代之时间.
代词 it 进行了以下的总结和归纳：
一.it 代指物体
1.A：What is this?B:It is a book.
2.A:What is taht?B:It is an apple.
3.A:Is this a dog?B:Yes,it is.
4.A:Is that a bag?B:No,it isn't.
5.A:Try it.You will get happy.
B:Really?Oh,it's so cool.
6.A:How does it work?
B:It's hard to say.But I will show you.
7.A:Say it again.B:Ok,listen.
二.it 代指人（未谋面的）
1.A:Who is calling?B:Oh,it is me,Tom.
2.A:Who is it calling?B:It's Mike calling.
3.A:Who's knocking at the door?
B:It's me,your brother.
4.A:Who's the man over there?
B:It's my friend,Peter.
三.it 代指时间
1.A:What time is it now?B:It is six sharp.
2.A:What day is it today?B:It is Sunday today.
3.A:What's the date today?B:It is May,1st.
4.A:When is your birthday?B:It is on Monday.
5:A:What time is it by your watch?
B:It is half past seven.
四.It 代指距离
1.A:How far is it from your house to the school?
B:It is ten miles.
2.A:How far is it from the downtown of the city to the village?
B:It is thirty minutes' drive.
3.A:How far is the park?
B:It's ten minutes' walk.
五.it 代指天气
1.A:What is the weather like today?
B:It is fine today.
2.A:How is the weather today?
B:It is getting cold.
六.it 代指事情
1.A:Let's talk about it.B:It's really boring.
2.A:Ok,just forget it.B:Not so easy as you think.
3.A:It is worth thinking about.
B:Yeah,I think so.
4.A:It won't take long.Just wait for me here.
B:Take your time.
5.A:Thank you very much.B:It's my pleasure.
6.A:I am sorry for that.B:It doesn't matter.
7.A:Think about it.B:I will.I will think it over.
8.A:You take it for granted.B:Certainly.He owes me a lot.
七.it 用于句型中,多做形式主语
1.Take it easy.
2.It is easy for sb.to do...
3.It is difficult for sb.to do...
4.It takes sb.+ 时间或金钱 to do...
5.It seems that .
6.It severs sb.right.
7.It is very kind of sb.to do...
8.It is said that...
9.It is reported that...
it 的用法属于英语语法中的“轻骑兵”,既不可轻视,又不要使其累赘你的学习.要在大量的阅读中重视它的用法,使其熟练起来.