Google calendar embedded code disappears

I’ve been trying to embed a Google calendar at the following address:
http://www.grassdirtcorn.com/calendar/

I’m running into the following problems:

1.) I switch over to HTM View, and put in the script. I switch over to Visual, and the script is gone and the calendar doesn’t show up.

2.) If I do #1, but hit “Save” before I switch to Visual, then the calendar DOES show up, however after a certain amount of time (hours, it seems), the code disappears again and the calendar is gone from the website.

How can I make a calendar actually stay put?