First of all, we're going to explore about at, on, and in for time
Compare at, on and in:

  • They arrived at 5 o'clock
  • They arrived on Friday
  • They arrived in  October / They arrived in 2018
We use:

We use at in expressions:

Note that we usually ask 'What time...?' (not usually 'At what time...?):
  • What time are you going out this evening?

  • We say: 
in the morning(s)                                 in the afternoon(s)                              in the evening(s)
  • I'll see you in the morning                            
  • Do you work in the evenings?
but:
on Friday morning(s)                        on Sunday afternoon(s)                    on Monday evening(s), etc
  • I'll be at home on Friday morning
  • Do you usually go out on Saturday evenings?
We do not use at/on/in  before last/next/this/every:
  • I'll see you next Friday. (not 'on next Friday)
  • They got married last March
In a few minutes/in six months etc. = a time in the future
  • The train will be leaving in a few minutes. (= a few minutes from now)
  • Jack has gone away. He'll be back in a week. (= a week from now)
  • She'll be here in the moment. (= a moment from now)
You can also say 'in six month' time', 'in a week's time' etc.:
  • They're getting married in six months' time. (=or...in six months)
We also use in... to say how long it takes to do something:
  • I learnt to drive in four weeks. (= it took me four weeks to learn)

Well, I guess you need also watch the video below to enrich your comprehension about preposition in, on, and at.

Okay, as practice makes perfect, let's exercise our brain now by taking the quiz below: