When will the day end when I check into the hotel at 9 am? Do I count the day at 6 pm or 9 pm, or at noon tomorrow 12?

The general hotel rule is to check out before noon 12 the next day. If it is exceeded, half a day's room charge will be added. But now you can generally discuss with the hotel in advance and postpone the check-out time to 14:00, without adding room charges. According to you, 6 pm or 9 pm, or tomorrow noon 12, is a day.