I am trying to include first and last selectors along with nextUntil and prevUntil
JQUERY
$('.selectedDay').parent().prevUntil('li.weeks').css('background', '#F00');
$('.selectedDay').parent().nextUntil('li.weeksClose').css('background', '#F00');
Try this:
$('.selectedDay').parent().prevAll('.weeks').first().css('background', '#F00');
$('.selectedDay').parent().nextAll('li.weeksClose').first().css('background', '#F00');