mywptrek

Subscribe your viewers to your calendar and inform them about your new events. 
Enable subscription to your events calendar. Inform your audience each time you create a new event.

Subscriber Addons for The Events Calendar

Subscriber Addons for The Events Calendar. Subscribe your viewers to your calendar and inform them about your new events through email.

mixailsayapin

Places on all pages (or selected) widget with a form to subscribe to the newsletter. 
The widget can be minimized. 
Remembers the form view. That is, if the user minimized the widget, it will be minimized until the user opens it himself. Therefore, it will not interfere with your visitors. 
Supports sending messages via SMTP. With this option, letters will surely be delivered to your recipients, because they are sent from your real mailbox, as if you did it from an email program. 
This plugin stores information about your clients in the database of your site, and does not transfer it to anyone, unlike many third-party services.

MS-Subscriber

Places on all pages (or selected) widget with a form to subscribe to the newsletter.

Subscriber Addons for The Events Calendar vs MS-Subscriber