These docs are for an old release.
Info on upgrading to v6
endParam
A parameter of this name will be sent to each JSON event feed. It describes the exclusive end of the interval being fetched.
String, default: 'end'
The actual value of this parameter will be an ISO8601 date string.
In line with the discussion about the Event object, it is important to stress that the value of this endParam date is exclusive just like elsewhere in the FullCalendar API.