Answer: The pope most often credited with establishing All Saints' Day for the entire Western Church on November 1 is Pope Gregory IV (827-844). During his pontificate, he extended the observance of the feast to the whole Western Church, making it a universal celebration.

However, the history is a little more complicated:
- Pope Boniface IV dedicated the Pantheon in Rome to St. Mary and All Martyrs in 609, and a feast honoring all martyrs was celebrated on May 13. - Pope Gregory III (731-741) dedicated a chapel in St. Peter's Basilica to all the saints and associated the celebration with November 1 in Rome. - Pope Gregory IV (827-844) then spread the November 1 observance throughout the Western Church, which is why he is often given credit for instituting All Saints' Day as a universal feast.

So if someone asks,"Which pope started All Saints' Day?", the best answer is:
Pope Gregory IV, who made November 1 All Saints' Day throughout the Western Church.

A fun historical note: the word"Halloween" comes from"All Hallows' Eve", the evening before All Saints' Day on November 1.