In a novel dripping with religious allegory and allusion, the seven day time frame shares the significance of the number seven, God's most perfect number in the Bible. Eco uses religious parallels within the seven day time frame to imbue The Name of the Rose with deeper religious symbolism and meaning.
The seven day time period is definitely suggestive of the seven days for God's creation of the world in Genesis in the Old Testament. The Creation story mirrors the development of the story; in the beginning when Adso and William first arrive at the library and monastery, the mystery begins and the main characters seem to be in total darkness, lacking understanding of the events around them.
More significantly, the seven days also parallel the seven trumpets, seven seals, and seven bowls of the Great Tribulation mentioned in the Book of Daniel and Revelation; the tribulation is a seven year time period in which God completes his Judgment, and the Antichrist reigns, and then is thrown down. Some of the mysterious deaths in The Name of the Rose share characteristics to God's judgment in the Seven Bowls that are poured out on the Beast's kingdom in the Book of Revelation.
First Bowl--Malignant ulcers
Second Bowl--Sea water turned to blood (Second murder: Venantius is found drowned in a vat of pig's blood.
Third Bowl--Rivers turned to blood. (Third murder: Berengar found in tub of water.)
Fourth Bowl--Scorching Heat (On the fourth day, Salvatore and Remigio are arrested by Bernard Gui, and the peasant girl for witchcraft...punishment for witchcraft is burning at the stake)
Fifth Bowl--The Beast's kingdom is plunged into darkness. (The darkness may be reminiscent of mistruth, as seen in the Inquisitorial methods and forced confessions. Severinius is found dead.)
Sixth Bowl--Euphrates dries up, so 200 million man army can gather for Armageddon (Malachi keels over dead during prayer, and it is clear that the murderer will stop at nothing to conceal the book.)
Seventh Bowl-- In the air, hail and lightning, earthquakes (Seventh day: Jorge confesses and the library burns down, Monk Benno is burned to death. )