Thankfully, Diane at Te Deum Laudamus has provided a description of the Rorate Mass. Relying on information given by one of the priests at Assumption Grotto in Detroit, Diane offers this history of the origin of the Rorate Mass:
The Rorate Mass has a long tradition in the Church, especially in German-speaking countries. It is a Votive Mass in honor of the Blessed Mother for the season of Advent. Our Lady shows herself in a special way as our leader through Advent to Christmas. The celebration by candle light had originally a more practical reason. According to the Missal of 1570 no Mass could be said after 12.00 Noon. On the other hand, people had to go to work in the morning. Also the Rorate Masses were celebrated in a more solemn form and therefore would last longer. For these reasons the Masses had to begin relatively early in the morning when it was still dark due to winter-time.I commend the entire post to you.