The coming of age for witches had always happened at the stroke of midnight.
This can be considered one of the unsolved mysteries of the witches. Even though magic power exists everywhere, there was no way to explain why it was the most active at this time. Even witches like Anna with immense magic reserves could entirely exhaust thier magic power and still fully recover within an hour or two after midnight. On the contrary, if the witches' magic power only recovered at the pace during the day, then even a week's time would probably still not be enough for them to get back to full strength.