Holy Monday is the day that Jesus went into the temple and began to cast out those who were buying and selling animals for sacrifice.
April 6, Cleansing the temple
“And He made a scourge of cords, and drove them all out of the temple, with the sheep and the oxen; and He poured out the coins of the money changers and overturned their tables.” Jn 2:15
Angered that worldly commercial activities had corrupted the holiness of the temple, He overturned the tables of the moneychangers, drove out the oxen and the sheep, and released the doves from their crates. The Gospels tell us that Jesus was ushering in a new covenant with humanity, in which animal sacrifice would end, replaced by the perfect sacrifice of His life on the cross to atone for human sin once and for all.