It was very long day for children 为什么要加IT WAS A VERY

It was very long day for children 为什么要加IT WAS A VERY
It was very long day for children
为什么要加IT WAS A VERY

It was very long day for children 为什么要加IT WAS A VERY
It was a very long day for children.才是正确的.
原因：
day是可数名词,前面要加不定冠词a.was也暗示了后面的名词为单数.
it was a （very) long day 表示“这是（很）漫长的一天”,a不能用the代替.

a very long day,泛指那一天是漫长的一天。day是可数名词，在前面通常要有定冠词特指某一天，或不定冠词或表示数量的词修饰。