<div id="calendar"></div>

document.addEventListener( 'DOMContentLoaded', () => {
  const calendarEl = document.getElementById( 'calendar' );
  const calendar = new FullCalendar.Calendar( calendarEl, {
    initialView: 'dayGridMonth',
    locale: 'local',
    themeSystem: 'bootstrap5',
  } );
  
  calendar.render();
} );