As today is the Feast of All Saints, here's some insight into how Catholics celebrated this feast day before the English Reformation, as described in Eamon Duffy's Stripping of the Altars.
November 1 and November 2 were important days on the Church calendar for Catholics in that era. They fasted on October 31, the vigil of the feast of all the saints in heaven. Halloween as we know it, is now a night for gathering candy and sweets! "Halloween" means the evening before All Hallows--All Saints. October 31st was thus one of the 70 days of fasting in the Church Calendar. All Hallows/All Saints was a Holyday of obligation then as it is today, meaning that Catholics were to attend Mass. Thus the term "Hallowmas" like Christmas, Michaelmas, etc.
As important as All Saints Day was because of the devotion of the people to their patron saints, All Souls Day was also important because it was the day set aside for praying for the dead who were not yet in Heaven. The Poor Souls in Purgatory were still undergoing their purification because of their attraction to sin during their time on earth. Their final destination was Heaven, but they were suffering in Purgatory--so their family on earth prayed for them, hastening their Heavenly happiness.