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$(".fancybox-trigger").click(function() {

    $("a[rel='" + $(this).data('trigger-rel') + "']").eq(0).trigger('click');

    return false;

});


$(".fancybox").fancybox();

<a data-trigger-rel="gallery" class="fancybox-trigger" href="http://fancyapps.com/fancybox/demo/1_b.jpg"><img src="http://fancyapps.com/fancybox/demo/1_s.jpg" alt=""/></a>

<br />
<br />

<a rel="gallery" class="fancybox" href="http://fancyapps.com/fancybox/demo/1_b.jpg"><img src="http://fancyapps.com/fancybox/demo/1_s.jpg" alt=""/></a>

<a rel="gallery" class="fancybox" href="http://fancyapps.com/fancybox/demo/2_b.jpg"><img src="http://fancyapps.com/fancybox/demo/2_s.jpg" alt=""/></a>