Today is the current day. Today begins with this morning. Then we have this afternoon, this evening, and tonight.
Yesterday is the previous day. Yesterday begins with yesterday morning. Then we have yesterday afternoon, yesterday evening, and last night.
Tomorrow is the following day. Tomorrow begins with tomorrow morning. Then we have tomorrow afternoon, tomorrow evening, and tomorrow night.
We have some other basic time phrases in English:
- in the morning
- in the afternoon
- in the evening
- at night
Exactly 12:00pm is noon.
Exactly 12:00am (o:00) is midnight.
The time period between 12:00am and about 4:00am is in the middle of the night.