n Rome, during the great persecutions, Mass was celebrated on the tombs of martyrs in the catacombs beneath the city, where the Christians fled for safety. The catacombs were underground galleries, of which it is said Rome had about 400 miles.

    This is the origin of the rule of having Mass said over relics of saints. At the beginning of the Mass the priest kisses the altar. By this, too, we, the Church Militant profess our communion with the saints in Heaven - the Church Triumphant. Often times on permanent altars, relics are kept in reliquaries on the altar flanking the candle holders.

    The lights which today we burn on the altar during Mass also had their origin during the times of persecution, when the Christians had to hear Mass in dark passages underground. They may be taken to symbolize divine grace.

Of the candles on the altar, two must be of pure wax. At a high Mass, at least six candles must be used. A sanctuary lamp of oil is kept burning day and night whenever the Blessed Sacrament is in the Tabernacle. For a Requiem Mass the white candles are replaced by waxed candles of a rose/lavender or grayish hue. This includes the floor candles around the bier at a Burial Mass.

Quote from: Charlemagne

Quote from: Vanessa

What do they represent and signify? Why are there 6?

From what I’ve heard (long ago), they represent the menorah, with Christ (the Crucifix) as the center light.

During a Pontifical High Mass, there are actually seven candles.  During a Low Mass, only two candles are lit, and during a Pontifical Low Mass, four candles are lit.

There are apparently many more regulations regarding the number of candles to be used during different liturgical functions than I previously thought:

Quote from: [i

Catholic Encyclopedia[/i], "Altar Candles"]Number of candles at mass

(1) At a pontifical high Mass, celebrated by the ordinary, seven candles are lighted. The seventh candle should be somewhat higher than the others, and should be placed at the middle of the altar in line with the other six. For this reason the altar crucifix is moved forward a little. In Requiem Masses, and at other liturgical services. e.g. Vespers, the seventh candle is not used. If the bishop celebrate outside his diocese. or if he be the administrator, auxiliary, or coadjutor, the seventh candle is not lighted.

(2) At a solemn high Mass, i.e. when the celebrant is assisted by a deacon and subdeacon, six candles are lighted. This is not expressly prescribed by the rubrics, but merely deduced from the rubric describing the manner of incensing the altar (Ritus celebrandi Missam, tit. iv, n. 4), which says that the celebrant incenses both sides of the altar with three swings of the censer prout distribuuntur candelabra.

(3) At a high Mass (missa cantata), which is celebrated without the assistance of deacon and subdeacon, at least four candles are required (Cong. Sac. Rit., 12 August, 1854), although six may be lighted. At these Masses under (l), (2), (3), the two lighted candles prescribed by the Missal (Rubr. XX) to be placed one on each side of the cross, are not necessary (Cong. Sac. Rit., 5 December, 1891).

(4) At low Mass celebrated by any bishop, four candles are usually lighted, although the "Caeremoniale Episcoporum" (I, cap. xxix, n. 4) prescribes this number only for the more solemn feasts, and two on feasts of lower rite.

(5) At a strictly low Mass celebrated by any priest inferior to a bishop, whatever be his dignity, only two candles may be used.

(6) In a not strictly low Mass, i.e. in a parochial or community Mass on more solemn feasts or the Mass which is said instead of a solemn or high Mass on the occasion of a great solemnity (Cong. Sac. Rit., 12 September, 1857), when celebrated by a priest more than two candles, and when celebrated by a bishop more than four candles may be used.

At all functions throughout the year except on Good Friday and Holy Saturday, before the Mass bishops are allowed the use of the bugia or hand-candlestick. The use of the bugia is not permitted to priests, whatever be their dignity, unless it be granted by an Apostolic privilege either personal, or by reason of their being curial dignitaries. If, on account of darkness a priest stands in need of a light near the Missal he may use a candle, but the candlestick on which it is fastened cannot have the form of the bugia (Cong. Sac. Rit., 31 May, 1817). An oil lamp can never be used for this purpose (Cong. Sac. Rit., 20 June, 1899). At the Forty Hours Devotion at least twenty candles should burn continuously (Instructio Clementina, section 6); at other public expositions of the Blessed Sacrament at least five (Cong. Sac. Rit., 8 February, 1879); at the private exposition, at least six (Cong. Episc. et Reg., 9 December, 1602). The only blessings at which lighted candles are prescribed are:

    of the candles on the feast of the Purification
    of the ashes on Ash Wednesday;
    of the palms on Palm Sunday.

There must be some reason, whether symbolic or otherwise, for the number of candles, but the above article doesn't mention any.