These docs are for an old release. Info on upgrading to v4
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refetchEventSources 2.8.0

Refetches one or more specific event sources.

.fullCalendar( ‘refetchEventSources’, sources )

The sources parameter can be a single identifier item, or an array of identifier items. An identifier item can be either an event source’s id/Array/URL/Function or the full Event Source Object.

Will result in a single re-rendering of events when all event sources are refetched.