I'm looking for a way to send email notification to all my users once my scheduled survey is already open. Upon checking the Webform module, all I can see about email notification is when a user submitted a response to the surey.
Webforms does not work that way. Webforms is for you or an administrator to receive an email when a user on your site fills out a form.
For example, if a customer wants you to send them a brochure in the mail, Webforms will let you know when they fill out your form.
If you want to notify multiple site users (not administrators) about a form being available, consider trying something like MailChimp, or one of the other popular mailing list programs. The well-known ones (MailChimp, Constant Contact, etc.) all have modules that will allow you to integrate them with your Drupal web site.