How can I make sure that when a form is submitted, at least ONE field is selected?

On this form, we want to list several events as separate questions so that they appear in separate columns when we download a submission report.

We need to make sure that the person submitting the form selects at least one event. Since these are separate "questions", we can't make them required.

is there a way to set up the form so that if none of the questions (events) are selected, a message appears on the form advising the user to select at least one event?

I thought I could do something with the form calculation widget, but have had no success so far. 

I checked your form. And I see you have correctly implemented the functionality you were asking for. So well done, great to see you managed to solve it yourself.

Should you require any further assistance, please just post another question. And we will be happy to help you.