请问on\at的区别

1 AT Christmas time,people give presents and send Christmas cards.
2 AT Easter,people celebrate the coming of spring.
3 Easter is also a religious holiday,and a lot of people go to church ON Easter Sunday.
4 ON Thanksgiving Day, people have a big dinner.
5 People have big parties ON New Year`s Eve.
6 ON New Year`s Day,everybody feels that he can make a new begining.
这几个句子都是指的节日，为什么有时候用AT 有时候用ON??
我很费解???

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