I've just experienced this problem.
It happened a week or so after updating to firmware v40.0.005. I now have no calendar on my homescreen nor any option to select it.
This thread came up when I searched for a solution. I'm pretty amazed to read that the problem has been going on for so long, and has not been addressed even in this very latest firmware version.
Got my calendar back using the Backup - Reset - Restore tip. Quite relieved as I didn't fancy hacking the software to get my phone working again as some other people have had to do.