CalendarEvent.php

<?php

class CalendarEvent
{
    /**
     * Holds the event
     *
     * @var string
     */
    protected $event;

    /**
     * Constants representing calendar event strings
     *
     * @var string
     */
    const EVENT_CHRISTMAS = 'christmas';

    /**
     * Construct method
     *
     * @param string $event
     */
    public function __construct($event)
    {
        $this->event = $event;
    }

    /**
     * Calculates time until event
     *
     * @return DateInterval
     */
    public function timeUntil()
    {
        $now = new DateTime();
        $eventDate = $this->toDateTime();
        return $eventDate->diff($now);
    }

    /**
     * Calculates time until event formatted as string
     *
     * @param string $format
     * @see http://php.net/manual/en/dateinterval.format.php
     *
     * @return string
     */
    public function timeUntilString($format = '%d days, %h hours, %i minutes, %s seconds')
    {
        return $this->timeUntil()->format($format);
    }

    /**
     * Converts CalendarEvent object to DateTime object
     *
     * @return DateTime
     */
    public function toDateTime()
    {
        $now = new DateTime();
        switch ($this->event) {
            case self::EVENT_CHRISTMAS:
                $eventDate = new DateTime(date('Y-m-d H:i:s', mktime(0, 0, 0, 12, 25, date('Y'))));
                if ($now > $eventDate) {
                    $eventDate->add(new DateInterval('P1Y'));
                }
                break;
        }
        return $eventDate;
    }
}

demo.php

<?php

require __DIR__ . '/CalendarEvent.php';

$christmas = new CalendarEvent('christmas');

while (true) {
    echo $christmas->timeUntilString() . "\n";
    sleep(1);
}

Found a problem with timeUntilString() when the event is more than a month away.