get_holidays_in_month.php

<?php
/**
 * Get count of holidays in a given month
 * Holidays are a content type with a date field
 *
 * @param      int  $month  The month
 * @param      int  $year   The year
 */
function cela_reports__working_hours__holidays_month($month, $year) {
  $holidays = db_select('field_revision_field_event_dates', 'f')
    ->fields('f', array('field_event_dates_value'))
    ->condition('entity_type', 'event')
    ->condition('bundle', 'holiday')
    ->execute()
    ->fetchAll();

  $holidays_this_month = array_filter($holidays, function($item) use ($month, $year) {
    return date('Y-n', strtotime($item->field_event_dates_value)) == date('Y-n', strtotime($year . '-' . $month));
  });

  return count($holidays_this_month);
}