At the end, by the end, and in the end
At the end (of)
At the end 可以指空间的尽头，时间、活动或故事的结尾。
Her office is at the end of the corridor.
The book has exercises at the end of every chapter.
The punctuation at the end of this sentence is a period.
at the end of the day
something that you say before you give the most important fact of a situation:
Of course I’ll listen to what she has to say but at the end of the day, it’s my decision.
at the end of your tether/rope
having no strength or patience left:
By six o’clock after a busy day I’m at the end of my tether.
By the end (of)
By the end 意为截止到某时间结束，在某时间结束之前，最迟到某时间结束。因此 by the end 不一定是在最后时刻。
The construction will be completed by the end of the year.
In the end
In the end，相当于“finally”，最终。指经过一系列考虑、讨论后，最终做出怎样的决策。
We were thinking about going to Switzerland, but in the end we went to Austria.
In the end, she chose to go to Oberlin College.