i had a very happy that day 语法和意思对吗？

语法不对～
“Very happy”是个形容词，所以后面一定要跟一个名词，不能有“that”。

真确的方法：
That day, I was very happy.
I was very happy that day.

（Day 就是名词，所以可以跟在 Very happy 的后面）

不对。

我那天很快乐！ ＝ I had a very happy day that day.

不能说 I had a very happy that day.

j'avais une très heureux ce jour-là

法语..

That day, I had a very happy day.

语法有问题，意思当然就有问题。
如果说I had a very happy day that day还勉强过得去，语法上没有大问题，但应该不会有人这么说。因为很拗口。
楼上这位的两种说法我觉得都挺好