I forget... is it a Holy Day of Obligation?
All Saints is the one that is a holy day of obligation. But much can be gained by praying for the souls in Purgatory.
Indulgences for All Souls Week
An indulgence, applicable only to the Souls in Purgatory, is granted to the faithful, who devoutly visit a cemetery and pray, even if only mentally, for the departed. The indulgence is plenary each day from the first to the eighth of November; on other days of the year it is partial.
A plenary indulgence, applicable only to the Souls in Purgatory, is granted to the faithful, who on the day dedicated to the Commemoration of All the Faithful Departed [November 2 {as well as on the Sunday preceding or following, and on All Saints' Day}] piously visit a church. In visiting the church it is required that one Our Father and the Creed be recited.
To acquire a plenary indulgence it is necessary also to fulfill the following three conditions: sacramental Confession, Eucharistic communion, and prayer for the intention of the Holy Father. The three conditions may be fulfilled several days before or after the performance of the visit; it is, however, fitting that communion be received and the prayer for the intention of the Holy Father be said on the same day as the visit.
The condition of praying for the intention of the Holy Father is fully satisfied by reciting one Our Father and one Hail Mary. A plenary indulgence can be acquired only once in the course of the day.