This is generally a Comcast customer peer to peer public forum. As such, the majority of users here cannot directly help you except to provide suggestions and common public information.
You should get notices via email or text alerting you to when your bill is due. You can also log into your account on the MyXfinity.com web site or use the mobile app Xfinity My Account to look at your billing preferences and detail.
Hello, Nessa3! Welcome to the forums. I get how important it is to ensure the bill is paid on time! You can view your bill online and it will show the due date. Here's how: https://www.xfinity.com/support/articles/viewing-your-bill-online-ecobill.
Please respond here if you have more questions!