Sanctus bells get their name from being rung first during the Sanctus [Holy, Holy, Holy Lord...]. A little before the consecration, when appropriate, a server rings a bell as a signal to the faithful. According to local custom, the server also rings the bell as the priest shows the host and then the chalice.
The reason for ringing bells is, first, to create a joyful noise to the Lord; second, the Church bells ringing signaled those not able to attend Mass that something supernatural was taking place. The use of bells in the Church dates back to the fifth century when they were introduced as a means to summon monks to worship. In the seventh century Pope Sabinianus approved the use of bells to call the faithful to the Mass.
It wasn't until the thirteenth century that outdoor tower bells began to be rung as "Sanctus bells" during the Mass. It is interesting to note that tower bells are still used today as Sanctus bells at the Basilica of Saint Peter in the Vatican and a great many other historic churches and cathedrals. A close look at many of these older structures will often reveal a series of sighting holes (and sometimes mirrors) that were once used by bell-ringers to monitor the celebration of the Mass from bell-lofts so that the bells could be rung at the proper time. (betcha didn't know that!)
Eventually, handheld bells, sanctuary-based chimes or "Gloria wheels" began to replace the tower bells rung during Mass -- largely for convenience.
Sanctus bells are traditionally kept on the epistle (left) side of the credence table during the Mass.
The bells are rung at three or four points during the celebration of the Mass:
1. Sanctus bells are first rung prior to the consecration at the epiclesis when the priest prays to the Holy Spirit to change the gifts of bread and wine into the Body, Blood, Soul and Divinity of Jesus Christ.
2. The bells are rung a second time as the priest elevates and presents the Body of Christ.
3. The bells are rung a third time as the celebrant elevates and presents the chalice filled with the Precious Blood.
4. The bells may be rung a fourth time as the priest-celebrant consumes the Precious Blood. This custom, which originated from the rubrics of the Tridentine Mass, may be continued since it is not forbidden nor suppressed in the latest version of the General Instruction of the Roman Missal.
Sanctus bells may also be rung at specified times outside of the Mass, such as during Holy Benediction and during adoration of the Most Blessed Sacrament.