This is a multi faceted question.
First of all, your web application cannot send emails at a set time, because your web app just sits there and waits for requests which are atomic ( self contained ). It has no state, no timers with which to fire an event.
Second, while your web app is on the web, it knows how to generate HTML for the web. Even if you wanted to send emails on request, it would still be irrelevant that your app is a web app, the answer to sending emails in C# is the same, no matter if it's a web app or a windows app.
This[
^] is the first of many articles I found on this site, on how to send an email in C#. As for how you do it, I would do it inside a windows service, as this can run on your server, all the time, and fire based on a timer.
Here[
^] is an article in creating a windows service.
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