KROPLJENJE GROBOV Z BLAGOSLOVLJENO VODO

 In his book Explanation of the Holy Sacrifice of the Mass  Fr. Martin Cocham says this about the pious custom you asked about:

"The suffering souls also experience a great alleviation when the graves are incensed and sprinkled with holy water. The holy water does indeed only materially moisten the earth, but in its beneficent virtue it refreshes the souls in Purgatory, just as the waters of Baptism falling on the head of the infant have power to cleanse his soul.

"Therefore see that you frequently sprinkle the last resting-places of your friends with holy water, but thereby you will allay the heat of the flames that torture them." (NY Benzinger Bros, 1896, p. 277)