add_to_several_calendars.js

function multiCalEventMaker () {
 
  var fromSS = 'Big Events, Small Events, Tiny Events'; //get calendars you want, could be checkbox entries from Google Form
  fromSS = fromSS.replace(/,\s+/g, ',').toLowerCase(); //gets rid of the space before the comma which ruined my life for too long
  Logger.log(fromSS); // 
  var calNames = fromSS.split(",");
//loops through however many calendars you set above
  for (i = 0; i <calNames.length; i++) {
   var calName = calNames[i];
   var calId = CalendarApp.getCalendarsByName(calName)[0]; //you'd change the next stuff to integrate whatever variables you want
     var event = CalendarApp.getCalendarById(calId.getId()).createAllDayEvent('Magic Stuff',
     new Date('March 25, 2016'),
     {location: 'The Moon'});
  }
  
}