It was on the National Day ____ she met with her separated sister.

问题描述:

It was on the National Day ____ she met with her separated sister.
此句为强调句,空格填that,那这句话改成什么形式,空格出可用when?

It was on the National Day【that】she met with her separated sister
比较:
It was the National Day 【when】 she met with her separated sister.