"The sacristan, who carefully arranges the liturgical books, the vestments, and other things necessary in the celebration of Mass" (GIRM, No. 105).

"The sacristan, always under the general direction of the clergy, undertakes the overall preparation of liturgical celebrations, including all that is needed for special days such as Ash Wednesday and Palm Sunday.

The sacristan thus arranges the books needed for the celebration, marking all of the divisions. He or she lays out the vestments and anything else needed for the celebration, such as cruets, chalices, ciboria, linens, oils, processional crosses, candles and torches.

He or she also takes care of the ringing of bells that announce the celebrations. The sacristan should ensure the observance of silence in the sacristy.

The sacristan in harmony with the pastor also makes sure that the vestments, church furnishings, liturgical vessels and decorative objects are kept in good condition and, if necessary, sent for gilding or repair.

Other practical indications apart from these official recommendations are that the sacristan ensures that the things necessary for worship are always available. There should be a ready supply of fresh hosts and of duly authorized wine, sufficient clean purificators, corporals, hand towels, incense and coals.

In this context the sacristan is responsible for making sure that those who wash the altar linens do so according to the indications of the missal and that the water for the first wash is poured down the sacrarium or to the earth. The sacristan also takes care of burning old linens and other objects that are no longer suitable for liturgical use.

He or she also makes sure that the sanctuary lamp has sufficient oil, that the altar cloths are changed regularly, and that the holy water stoups are clean and replenished frequently.

The pastor may also decide to entrust other responsibilities to the sacristan. This might include coordinating others who help with the general decor of the church, such as cleaners and flower arrangers. The sacristan might also maintain the practical dealings with external agents such as funeral directors and photographers so that proper decorum is maintained at all times.

In order to carry out these duties, the sacristan needs to have a fairly good idea of the content and norms of the principal liturgical books and an understanding of the intricacies of the liturgical calendar.

A good sacristan is a boon to any parish and, as the GIRM says, the post fulfills a true liturgical function. As the Ceremonial of Bishops states: 'The adornment and decor of a church should be such as to make the church a visible sign of love and reverence toward God' (No. 38)."
(exerpt taken from  A Sacristan's Duties | EWTN)