When you make a vow to your Patrons that you're going to fast on the Full Moon, you will fast on the full moon, even when you forget what day it is and eat something quick. Well, at least that's true for me. I have been sick all day. I have an awful fever, nothing is digesting and I can't even look at food. Oops. Guess I need to send myself an e-mail reminder about the full moon date. I can't type much, so I'm going to C&P something from http://www.religioromana.net/onsacrifices.htm They have wonderful information on their site and try to replicate Religio Romana as closely as possible. I admire their research.
The six times sacrifices are made
There are six types of sacrifices that may be made in the Religio Romana.
1) First is to honor the gods and to commemorate certain events such as the dedication of a temple.
2) To propitiate the gods when some disaster has occurred or other event whereby the gods demand a sacrifice.
3) Similarly, if divination, dreams or visions reveal a requirement that a sacrifice needs to be made.
4) By far though, most sacrifices made by individuals are performed in thanks, after a contract was met by the gods. A vow (nuncupatio) is first made that a sacrifice will be offered, or that an altar will be erected, or a temple built or renovated, or some other action that will be taken to fulfill the vow, on condition that the god perform some request. If the request is fulfilled, the deity having accepted the vow, then one is obliged to fulfill the vow (ex voto); failure to do so would make that person sacer. Sometimes,
5) in expectation that prayers will be answered, a sacrifice is made, but there is no obligation on the part of the deity. Most often these types of sacrifices would be dedicated pro salute in hopes of being healed of some illness.
6) Lastly there are those sacrifices made, not to honor the gods or to fulfill a vow, but made instead as part of a purification rite.