The story
The dead are held to come back for the night, and the house prepares for guests rather than for grief. An altar is built with their photograph, and with what they liked to eat and drink.
Marigolds are laid in a path from the door because their smell is said to be what the dead follow. Water is set out because the journey is long, and salt, and a candle for each person expected.
It is not Mexican Halloween, and it is older than either name: the church calendar was set over an existing observance rather than replacing it.