Here is a simple way to add a Google Calendar to your WordPress page or post. Instead of trying to add an iFrame, this plugin will allow you to use a shortcode instead. Giving you a more reliable calendar.
How it works
First we need to create the plugin and call in the one required file
Then we need to create the shortcode “pydgooglecal” and allow it to pull in some options
This plugin just covers the basic parameters, but you can add more options to fit your needs. Just remember to urlencode them before sending it into the iFrame.