When I had a bad day I say "I had a bad day.", but my friend said "I have a bad day.".
Technically the day ends at 0 o'clock, but I think past tense is more suitable because we are talking about something already happened.
Which one is more natural to say at night?
"I had a bad day" or "I've had a bad day" are both possible at night. The first one makes it clear that the day is over, whereas the second doesn't.
"I have a bad day" is not possible, even during the day, but "I'm having a bad day" is possible. If a person said "I have a bad day," it would mean that he repeatedly has bad days, as in "Every time I get up early in the morning, I have a bad day."
