请问on\at的区别1 AT Christmas time,people give presents and send

请问onat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?