Days of the week
All days of the week begin with a capital letter.
I will call you on Monday.
I met her on a Friday.
Note that we use the preposition on with days of the week. In an informal style, on can be left out. This is common in American English.
I am seeing your Dad Sunday evening. (OR I am seeing your Dad on Sunday evening.)
When we talk about repeated actions, we use the plural forms Sundays, Mondays etc.
I usually go and see my grandparents on Sundays.
To talk about parts of the day, we use the preposition in.
Note the expressions:
In the morning / in the evening / in the afternoon
I get up early in the morning.
She works best in the evening.
The meeting is at 3 o’clock in the afternoon.
There is a difference between in the night and at night. In the night means during one particular night. At night means during any night.
I got up several times in the night. (Perhaps during the previous night)
I hate to work at night.
We use on if we say which morning or afternoon we are talking about.
I met her on a cold evening in December.
See you on Sunday morning.
We use at the talk about the whole of the holidays at Christmas, New Year and Easter. But we use on to talk about any one day of the holiday.
See you on Christmas day.
In British English, at is used with weekend. In American English, on is used.
I had nothing to do at the weekend. (GB)
I had nothing to do on the weekend. (US)