At/on/in - time

Compare at, on and in:

• They arrived at 5 o'clock.
• They arrived on Friday.
• They arrived in October or in 1998.

We use at for the time of day:

• at five o'clock
• at 11.45
• at midnight
• at lunchtime
• at sunset

We use on for days and dates:

• on Friday/Fridays
• on 16 May 2009
• on Christmas day
• on my birthday

We use in for longer periods, for example: months/years/seasons

• in October
• in 1998
• in the 18th century
• in the past
• in (the) winter
• in the 1980s
• in the Middle Ages
• in (the) future

We use at in these expressions:

• at night - I don't like working at night
• at the weekend/ at weekends - Will you be here at the weekend?
• at Christmas - Do you give each other presents at Christmas?
• at the moment / at present - The manager isn't here at the moment/ at present.
• at the same time - Kate and I arrived at the same time.

We say:

• in the morning(s) - but on Friday morning(s)
• in the afternoon(s) - but on Sunday afternoon(s)
• in the evening(s) -  but on Monday evening(s)
• I'll see you in the morning - but I'll see you on the Friday morning
• Do you work in the evenings? - but Do yo work on Saturday mornings? 

We do not use at/on/in before last/next/this/every

• I'll see next Friday (not on next Friday)
• The got married last March.

In spoken English we often leave out on before days (Sunday/Monday etc.). So we can say:

• I'll see you on Friday or I'll see you Friday.
• I don't work on Monday mornings. or I don't work Monday mornings.

In a few minutes / in six months etc.

• The train will be leaving in a few minutes. (= a few minutes from now)
• Andy has gone away. He'll be back in a week. (= a week from now)
• They'll be here in a moment. (a moment from now)

We can also say: in six months' time or in a week's time etc.

• They're getting married in six months' time / 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 to drive)