| 1. + time of the day
at 2 o'clock
at midnight
at noon (= 12 o'clock in the daytime)
at lunchtime
2. + weekends
at the weekend
at weekends
3. + public holiday
at Christmas;
at Easter
| 1. + a part of the day
in the morning
in the afternoon
in the evening
BUT: at night
2. + longer periods (months, seasons, years, etc.)
in July;
in (the) summer;
in 1998;
in the sixties;
in the 20th century
| 1. + a day
on Monday
on Wednesday
on Saturday
on Christmas day
2. + a day + a part of the day
on Monday morning
on Saturday night
on Wednesday evening
| 1. before next, last, this
next week
last night
this year
2. before today, tomorrow, yesterday, tonight
|