Just wrap the modal you want to call on page load inside a jQuery load event on the head section of your document and it should popup, like so:

JS

<script type="text/javascript">
    $(window).on('load', function() {
        $('#myModal').modal('show');
    });
</script>

HTML

<div class="modal hide fade" id="myModal">
    <div class="modal-header">
        <a class="close" data-dismiss="modal">×</a>
        <h3>Modal header</h3>
    </div>
    <div class="modal-body">
        <p>One fine body…</p>
    </div>
    <div class="modal-footer">
        <a href="#" class="btn">Close</a>
        <a href="#" class="btn btn-primary">Save changes</a>
    </div>
</div>

You can still call the modal within your page with by calling it with a link like so:

<a class="btn" data-toggle="modal" href="#myModal">Launch Modal</a>