I remember the day I got my first paycheck.

= I remember the day when I got my first paycheck.

= I remember the day that I got my first paycheck.

= I remember the day on which I got my first paycheck.

当定语从句的先行词是：

the way （方式）

the reason （原因）

the time/day （时间）

the place （地点）

后面的定语从句，可以使用关系副词 that 引导或者省略that ，还可以使用相应的关系副词或“介词+which“，例如：

Please tell me the reason you did it. 

Please tell me the reason that you did it.

Please tell me the reason why you did it.

Please tell me the reason for which you did it.