[Sebastian - in his ever-present, usual uniform - probably nagged Ciel into attending (because while Ciel doesn't like parties, Sebastian generally does, and more importantly he enjoys forcing the young master to do things the young master doesn't like).
Sebastian kind of regrets it as they enter, and he spots Lucifer by the piano. But he looks around the ballroom and... actually looks surprised. He can't find anything to criticise. It looks good.]