When the Romans conquered Celtic territories from A.D. 43 onwards, they brought with them their own traditions and festivals which began to merge with the existing Celtic customs, including Samhain.
One of these Roman traditions was the festival of Feralia, a day in late October when the Romans traditionally commemorated the passing of the dead. The celebration of Feralia was intended to honor the spirits of deceased ancestors and involve making offerings at graves, including a variety of food and drink dedicated to the spirits of the dead.
The other was a day dedicated to Pomona, the Roman goddess of fruit and trees. Pomona’s symbol was the apple, which could explain why bobbing for apples became a tradition at Halloween. It's believed that the incorporation of these festivals into Samhain was part of the Roman effort to gradually introduce their culture and beliefs to the Celtic tribes.
The blend of Samhain's emphasis on the supernatural with Feralia's commemoration of the deceased and Pomona's celebration of the harvest created a new festival that retained elements from both cultures. Over time, this fusion of cultures and their festivals would evolve, paving the way for the Halloween celebrations we know today.