Translation of le reveillon apres la messe?

You are probably talking about Christmas here. The French tradition at Christmas is to go to Mass at midnight (Christmas Eve) and then have a celebratory meal afterwards. Un réveillon means you stay up and don't go to bed until the wee hours - the verb is 'réveillonner'. French people also 'réveillonnent' on New Year's Eve. The verb 'veiller' also exists and also means to stay up all night and 'la veille' means the eve or the evening that precedes an important day. This idea of the eve goes back to the Middle Ages, when it was accepted that when nightfall came, that day had ended and the ensuing night belonged to the following day, which is why we celebrate All Saints' Day (Hallow e'en or evening) from nightfall the preceding evening. So 'le reveillon aprés la messe' is the late night feast after Mass.