That night or On that night? i have problems with the preposition...
That night, all of them were very happy.
On that night, all of them were very happy.

which one is correct

Question

That night or On that night? i have problems with the preposition...
That night, all of them were very happy.
On that night, all of them were very happy.

which one is correct

Answer

Both are correct.
The word "on" focuses attention on that night as a special event. 
In the first sentence, they were happy for some reason. In the second, it's likely that there was some feature of that night that made them happy.