On this day the Church celebrates a solemn feast for all the deceased who, having passed purgatory, have been totally sanctified, have obtained the beatific vision and enjoy eternal life in the presence of God. That is why it is "All Saints' Day." It is not celebrated only in honor of the blessed or saints who are on the canonized list and for whom the Church celebrates on a special day of the year; it is also celebrated in honor of all those who are not canonized but already live in the presence of God in his triumphant Church.
On this day, the great cathedrals often display the relics of the saints. The early Church used to celebrate the anniversary of the death of a martyr at the place of martyrdom. Groups of martyrs often died on the same day, which naturally led to a common celebration. In the persecution of Diocletian, the number of martyrs became so great that a day could not be set aside to assign it to them. But the Church, believing that each martyr should be venerated, designated a common day for all.
With much nostalgia we like to remind our loved ones that they are no longer by our side. This is our way of honoring them all.
Ref: https://es.wikipedia.org/wiki/D%C3%ADa_de_Todos_los_Santos