1. The solemn exposition of the Most Holy Sacrament consists in placing the sacred Host in a monstrance, unveiled, at some height above the altar for the adoration of the faithful.
In the early ages of Christianity the Blessed Sacrament was concealed as much as possible from sight, lest the unbaptized might conceive contempt for the Christian mysteries. Public exposition was not introduced until after the institution of the feast of Corpus Christi. The number of lighted tapers must not be less than twelve. The mere opening of the tabernacle is not a solemn exposition; every parish priest may do that on his own authority.
2. Exposition of the Blessed Sacrament generally takes place on these occasions: After the parochial Mass, at the afternoon services on Sundays and festivals, on Holy Thursday on the altar of repose, on the feast of Corpus Christi. The bishop of the diocese often orders an exposition to be held for some reason of local or general interest; such as a public calamity, the dangerous sickness of the ruler of the land or of the Holy Father, etc.