Depends how you define as the start of the day. If I define midnight as the start of a new day, it's dark so night comes first. Or you could define the start of the day at sunrise, so daytime would come before night :thinking:
It's pretty obvious that the day starts at 6:00am and if you are out doing anything you will be sent back to your house and Isabelle will welcome you to a new day.
I generally consider it to be the next day after I've slept or once it gets light/wake up. For example, if I said I'll do something tomorrow, I wouldn't expect anyone to expect me to do it some time during the night, therefore "tomorrow" comes when it gets light or when I wake up.