那天是香港回归的日子用英语怎么说

That day was the day for The Return of Hong Kong

That day was the day when Hong Kong returned to China.

或That day was the day of the Return of Hong Kong.

如果说不是回归当天的话，而是周年日的话：
That day was/is the anniversary of the Return of Hong Kong.

二楼和三楼应说That day is the one......好些。一楼的The motherland China.用的很好。

That day is the one on which Hong Kong returned to the mothland,China.