at night:在夜里,夜晚 (副词)
eg.We can see the stars at night.
夜晚我们可以看见星群.
in the night:在夜间
eg.I woke up twice in the night.
我在夜里醒了两次.
(at night=in the night,这两个没有区别)
on a fine night
eg.On a fine night like tonight,I'd rather go cruising than stay at home.
像今晚这么好的夜色,我宁愿开车到处闲逛也比待在家里好.
一般没有at the night 的说法