Event Reminders are set by the publisher (i.e.: the Brand, Organisation, or Team etc.).
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You may be able to adjust these notifications within the entry itself on some calendar types e.g Google Calendar or you may be able to turn all notifications off, again depending on your chosen calendar application.
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If subscribed to publisher’s events that are feed driven (automated) and you have manually changed the reminders, these will automatically revert back when the event content is refreshed and updated.

Information for turning off all notifications for each calendar type is available in the support hub.