We use:
IN
Before parts of the day,, months, years, and seasons
Examples:
- Gustavo enjoys reading in the afternoon.
- My wedding is going to be in August.
- I started my blog in 1999.
- I love going to the beach in summer.
Before days, and dates
- The party will be on January 27th
- Miguel's birthday is on Sunday
At is used with noon, night, midnight, midday, and with the time of day (also for the weekend but this is used in British English:
- My father comes from work at noon.
- My favorite TV program starts at 6 p.m.
- I don't eat at night.
Other prepositions
Extended time
Other commons prepositions to express time are: by, from—to /from-until, during,(with)in, last, next, every.
- The studied in that institute from January to (until) March. (Beginning in Januaryand ending in March.)
- I was watching TV during the evening. (For some period of time in the evening.)
- We should get married within a year. (No longer than a year.)
- I went to a party last night (anoche)
- I take a shower every morning (cada)
