Currently, it is not possible to manually send event announcement or reminder e-mails. They are only sent automatically if they are enabled on the Event E-mails tab of the Team Settings page
. Please note that if an announcement or reminder e-mail has already been sent, a new one will be sent for an event if the date, time, or location for the event is updated.
On a related note, it is possible to send a team e-mail in context of a specific event. Please see this FAQ