在第一天用英语怎么说

"On the first day" 的英文表达方式是 "on the first day"。这是一个常见的英文表达方式，用来指第一天。
例如：
On the first day of school, I was nervous about meeting my new classmates.
On the first day of the new year, many people make resolutions to improve their lives.
On the first day of our vacation, we went to the beach and had a great time.
在这个例子中，"on the first day" 用来指明事情发生的具体时间。