用 that。

这样时间名词： 

the first/second time

the first/second  day

the first /second week

the first/second  month

the first/second year 

the last time

the moment,

the minute 

为先行词时，都是使用that，不用 when。

这些词语时间概念明确，知道是指时间，而且that可以省略。

 在It is/ was the first/second/last time that …句型中，that后的从句谓语用

现在/过去完成时。偶尔也用一般过去时。

This is the first time I have traveledby air.

“Do you know the city well?” “No, this is the first time I have come here.”

关系副词when 在定语从句中作状语，表示时间,  它们的先行词分别是表示时间的名词。

如：

There are moments when I forget all about it.

People are still talking about the historic event of years ago, when man walked in space for the first time.  

关系代词that 在定语从句中作宾语或主语。

He will never forget the days that he spent with his grandma.

用 that。