Holy Crypt! Days of the Dead October 31-November 2

The term "Halloween" is a contraction of "Hallowed" (holy) "evening", the evening before the liturgical feast of All Saints on November 1.  The Solemnity of All Saints in honor of Early Christian martyrs and other holy individuals was in place by the Middle Ages.  The pagan feast of Samhain, the Celtic Lord of Death, also began on that date, and was held to be the time when the souls returned to earth to haunt those who had done them ill.  The Roman Feralia, a celebration of the dead in late October, also might have influenced the development of cult practices to honor and appease the dead that are preserved in very carnivalesque form in today's Halloween.  In the Roman Catholic Church, All Saints is a Holy Day of Obligation.  
The following day, November 2, is the Feast of All Souls, or Commemoration of All the Faithful Departed.  All the dead are prayed for, and in some countries, traces of pre-Christian ancestor veneration remain, such as the decoration of tombs with flowers and the laying out of food for the deceased.  
November 1 Masses are offered in churches connected to catacombs (for example, San Sebastiano, Santi Marcellino e Pietro), but the Pantheon is perhaps the most symbolic site for this important Christian feast, a pagan temple consecrated in 609 as a church to Mary and All Martyrs.
Happy Holiday(s)!