When a show's starttime is after midnight, it technically occurs on the next day.
The In The Night switch gives you the option to make this visible in your Calendar and iCal / Google Calendar.
Note: this does not effect your Dashboard.
For example:
Your artist is playing a festival on Friday, and the starttime of their show is 0:30 AM.
Technically the show occurs on Saturday 0:30 AM.
On the contract you want to display Friday 0:30 AM.
In the Calendar you want to display Saturday 0:30 AM.
To achieve this, you can set the "In the Night" switch for this in the event tab or you can set it while adding a new booking.
Your ABOSS Calendar and iCal / Google Calendar will show the correct date (in this case, the 23rd of December) of the event in the overview and will inform you in time when alerts are set.
That is basically what the in the night option does within ABOSS.