nextDayThreshold

How far an event must run into the next day before it renders on that day. It applies when an event’s end time spans into another day.

Only affects timed events that appear on whole-days: in DayGrid views, multi-month views, list views, and Timeline views with day-sized slots. It does not affect all-day events, or timed events in TimeGrid views, which display against a time-of-day axis.

For example, if nextDayThreshold were '09:00' (9am), the following event would appear to take up only one day:

const calendar = new Calendar(calendarEl, {
  // other options...
  nextDayThreshold: '09:00:00',
  events: [
    {
      // Goes from 8pm to 2am the next day.
      title: 'Event 1',
      start: '2014-02-04T20:00:00',
      end: '2014-02-05T02:00:00'
    }
  ]
});

Whereas, the following event would appear to take up two days:

const calendar = new Calendar(calendarEl, {
  // other options...
  nextDayThreshold: '09:00:00',
  events: [
    {
      // Goes from 8pm to 10am the next day.
      title: 'Event 2',
      start: '2014-02-04T20:00:00',
      end: '2014-02-05T10:00:00'
    }
  ]
});

It is important to stress that this nextDayThreshold option, set on the Calendar itself, is ignored when allDay is true. Generally, as discussed when defining an Event object, allDay, start and end properties are sufficient for specifying an event that lasts an entire day.

However, when allDay is false, nextDayThreshold sets the cut-off point (or minimum) for the event to be rendered on the day its end falls on. The event’s actual end remains exclusive; only which days it is displayed on changes.