The Rorate Mass is a traditional Mass held during Advent, usually early in the morning (pre-dawn) or late in the evening, where the entire church is lit only by candlelight. Mass begins with the singing of the Rorate Cæli (translated "Drop down dew, ye heavens"), and as it is a votive Mass for the Blessed Virgin Mary, white vestments are worn instead of the typical Advent violet.
In St. Louis, we had a Rorate Mass at St. James the Greater parish in Dogtown, which is a western part of the City of St. Louis (and about a 10 minute drive from my house).
Like Tenebræ, It's one of the liturgical events that's better experienced in person than viewed through the lens of a camera, but I did as much as I could with the dim candlelight available and my f/2.8 lenses (most shots were handheld):
See all the photos from the Rorate Mass on Flickr.