The Salian Order

     The Salian order or Salii (meaning jumping priests) consisted of twelve priests dedicated to Mars Gradivus and twelve priests dedicated to Quirinus. Its members were chosen from patrician families and both their parents had to be alive. They were appointed for life, though they were allowed to resign from the Salian priesthood if they achieved a major magistracy or a more prestigious priesthood (the Salian order was considered a lesser priesthood). The priests of Mars himself were called the Flamen Martialis. The Salian order dated from well before the republic and was said to have been instituted by the second Roman king Numa Pompilius. The priests dedicated to Quirinus were later instituted by Tullus Hostilius in fulfillment of a vow which he made in a war with the Sabines. In early Roman history Mars was a god of spring, growth in nature, and fertility and the original rituals performed by the Salii reflected this. They were intended to protect the growth of plants with the coming of spring in March. In later times the rituals of the Salii signified the starting and ending of the war season which lasted from March till October. Like so many gods Mars evolved with Roman needs from the early Republican era, when Rome was an agricultural city to the times when Rome developed into a city of commerce and military force. And thus the order became more associated with Mars Gradivus ("he who precedes the army in battle"). Plutarch (46 - 126 AD) a Greek historian, biographer, and essayist wrote the following about the origin and customs of the Salian order :

The origin of the Salii is this. In the eighth year of the reign of Numa, a terrible pestilence, which traversed all Italy, ravaged likewise the city of Rome; and the citizens being in distress and despondent, a brazen target (edit: target is an ancient word for shield), they say, fell from heaven into the hands of Numa who gave them this marvelous account of it: that Egeria and the Muses had assured him it was sent from heaven for the cure and safety of the city, and that, to keep it secure, he was ordered by them to make eleven others, so like in dimension and form to the original that no thief should be able to distinguish the true from the counterfeit. He farther declared, that he was commanded to consecrate to the Muses the place, and the fields about it, where they had been chiefly wont to meet with him, and that the spring which watered the field should be hallowed for the use of the vestal virgins, who were to wash and cleanse the penetralia of their sanctuary with those holy waters. The truth of all which was speedily verified by the cessation of the pestilence. Numa displayed the target to the artificers and bade them show their skill in making others like it; all despaired, until at length one Mamurius Veturius, an excellent workman, happily hit upon it, and made all so exactly the same that Numa himself was at a loss, and could not distinguish. The keeping of these targets was committed to the charge of certain priests, called Salii, who did not receive their name, as some tell the story, from Salius, a dancing-master born in Samothrace, or at Mantinea, who taught the way of dancing in arms; but more truly from that jumping dance which the Salii themselves use, when in the month of March they carry the sacred targets through the city; at which procession they are habited in short frocks of purple, girt with a broad belt studded with brass; on their heads they wear a brass helmet, and carry in their hands short daggers, which they clash every now and then against the targets. But the chief thing is the dance itself. They move with much grace, performing, in quick time and close order, various intricate figures, with a great display of strength and agility. The targets were called Ancilia from their form; for they are not made round, nor like proper targets, of a complete circumference, but are cut out into a wavy line, the ends of which are rounded off and turned in at the thickest part towards each other; so that their shape is curvilinear, ..... The reward which Mamurius received for his art was to be mentioned and commemorated in the verses which the Salii sang, as they danced in their arms through the city; though some will have it that they do not say Veturium Mamurium, but Veterem Memoriam, ancient remembrance.

The Ancilia carried by the Salii (from  Smith, Dictionary of Greek and Roman Antiquities)

     The Ancilia were kept at the sacrarium Martis in the Regia on the Forum Romanum with the 'hastae Martiae' ("lances of Mars") together they formed the symbolic armory of the Roman state. On the first of March the Salii went to the sacrarium and retrieved the Ancilia and carried them trough the streets in procession as described above. At the end of each day the procession stopped at a certain house where the shields were kept for the night, and the Salii took part in a lavish banquet. These processions lasted till the 24th of March when the shields were once again placed in the sacrarium. Near the end of the year the Salii performed again during the Armilustrium which was held on the 19th of October. The Armilustrium marked the end of the campaigning season, the weapons and armor of the soldiers were ritually purified and stored for winter.

     During their processions the Salii sang a hymn called the Carmen Saliare or Axamenta only a few lines of this hymn are left to us. They are written in an archaic form of Latin so old that by the middle of the first century BC the song had become almost unintelligible even to the priests themselves. It appears that the praises of Mamurius Veturius formed the principal subject of these songs, and that they also contained the praises of all the celestial deities, with the exception of Venus. The verses in honour of each god were called by their respective names Januli, Junonii, Minervii. Later as a sign of divine honour the names of several emperors were incorporated in the songs of the Salii. This honour was first bestowed upon Augustus later upon Germanicus and when Verus died, his name was inserted in the song of the Salii by command of Marcus Aurelius.
The fact that the Romans took the rituals performed by the Salii very seriously can be seen by this description from Polybius (203 - 122 BC) in his book The Histories (book 21 verse 13 lines 10-14) on Scipio Africanus who was a member of the Salian order:

The real reason why both the army remained in its first camp and Scipio was apart from it was that the latter was one of the Salii. These are, as I said in my book on the Roman constitution, one of the three colleges whose duty it is to perform the principal sacrifices, and, no matter where they happen to be, it is forbidden for them to change their residence for thirty days during the celebration of the sacrifices. This was now the case with Scipio; for just as his army was crossing, he was caught by this period, so that he could not change his residence. The consequence was that he was separated from his army and stopped behind in Europe, while the legions after crossing remained inactive, and were unable to make any progress as they were awaiting his arrival.

For Scipio it was a case of either being in Rome, helping to carry the Ancilia, or staying put for 30 days so as not to profane the ritual even if it meant halting a military campaign.