Workflow rules on recurring events and tasks
|Knowledge Article Number||000187951|
Workflows that operate on Recurring Events or Tasks will only modify the individual task record, not the master series page.
If you wish to modify the master series page, you should use an Apex trigger (before insert and/or before update) on the Event or Task object, and use the "isRecurrence" field to limit your modifications. Note that modifications to the series page will be propagated to the task record automatically.