One workaround is to use radio buttons instead of a checkbox. You can use the FP validation on a radio button. Since you offer multiple selections you would need a radio button "group" for each event.

A bit lame- if I select an event by accident or change my mind there isn't any way to uncheck that radio button.

Another thing to do is put a onMouseDown event handler on the submit button and run a validation script.

In the script make use of something like:
var valid=false;
if(check1.checked==true)
{
valid=true;
}

//make a condition for each box
//or make some sort of loop to check each box

if(valid==true)
{
document.formName.submit();
}
else
{
alert("Check at least one of the options");
}