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Programmatically generates HTML that will be injected on the calendar’s column headings.

function( date ) -> string

This function will will receive a date object that represents a single column’s date and must return an HTML string to be injected.


var calendar = new Calendar(calendarEl, {
  columnHeaderHtml: function(date) {
    if (date.getUTCDay() === 5) {
      return '<b>Friday!</b>';
    } else {
      return '<i>other day</i>';