"There are no calendars selected" error message

I recently had this error message appearing from time to time on my phone and I had no ideas why/what/how. I am not using calendars too much so I did not care too much about the problem but the error message was annoying. What was I supposed to do? Which application was involved if any? I looked on the Internet and began to panic a bit. Some people were advising to factory reset the phone and reinstall the apps one by one and see if the problem appeared again... Fortunately, I was not happy with this solution and found that in my case, the problem was easy to solve.

My problem was coming from the app "Business Calendar" and this is what I needed to do to fix it:
1) Launch the app (if you have the calendar on your desktop, just click on it)
2) A calendar should appear. Click on the option button (your android phone button)
3) You should have the following options: Agenda, Search, Today, Go to, Show Bars and More. Choose More
4) Select the first entry called "Calendars"
5) Check the check box corresponding to the calendar you want to use

6) Save
7) Enjoy

Notice: the problem may come from something else on your phone, so I cannot guarantee that this will fix your problem. At least it did for me.