A. We use adverbials of time to say. for how long, how often (frequency)
A1. We often use a noun phrase as a time adverbial: yesterday, last week/month/year, one day/week/month, last Saturday, tomorrow, next week/month/year, the day after tomorrow, next Friday, today, this week/month/year, the day before yesterday, the other day/week/month
B. We use phrases with prepositions as time adverbials:
B1. use ‘at’ with: clock times: at seven o’clock; at nine thirty; at fifteen hundred hours, and mealtimes: at breakfast; at lunchtime; at teatime, and in these phrases: at night; at the weekend; at Christmas; at Easter
B2. use ‘in’ with seasons of the year: in spring/summer/autumn/winter, years and centuries: in 2009; in 1998; in the twentieth century, and months: in January/February/March etc., and parts of the day: in the morning, in the evening.
B3. use ‘on’ with: days: on Monday/Tuesday/Wednesday etc, and on Christmas day; on my birthday, and dates: on the thirty first of July; on June 15th
C. We say at night when we are talking about all of the night.
C1. but we say in the night when we are talking about a short time during the night.
C2. We use the adverb ago with the past simple to say how long before the time of speaking something happened.