title: Execute a code at some point of time
body: <p>I am creating a web app made of python flask.. What I want is if I created an invoice it automatically send an e-mail on its renewal date to notify that the invoice is renewed and so on.. Please take note that renewal occurs every 3 months The code is something like:</p>
  
  <pre><code>def some_method(): // An API Enpoint that adds an invoice from a request
      // Syntax that inserts the details to the database
      // Let's assume that the the start of the renewal_date is December 15, 2016
  </code></pre>
  
  <p>How will the automatic email execution be achieved? Without putting too much stress in the backend? Because I'm guessing that if there are 300 invoices then the server might be stressed out</p>