These docs are for an old release.
Info on upgrading to v4
nextDayThreshold
When an event’s end time spans into another day, the minimum time it must be in order for it to render as if it were on that day.
Duration, default: "09:00:00" (9am)
Only affects timed events that appear on whole-days. Whole-day cells occur in month view, basicDay, basicWeek and the all-day slots in the agenda views.
For example, with nextDayThreshold being the default of 9am, the following event would appear to take up only one day:
{ start: '2014-02-04T20:00:00', end: '2014-02-05T02:00:00' }
// goes from 8pm to 2am the next day
Whereas the following event would appear to take up two days:
{ start: '2014-02-04T20:00:00', end: '2014-02-05T10:00:00' }
// goes from 8pm to 10am the next day