The Town That Dreaded Sundown

Before the "Phantom-attacks", which occurred about eight months after World War II, Texarkana was pleasant and citizens were preparing for a good future. On the night of Sunday, March 3, 1946, Sammy Fuller and Linda Mae Jenkins park on a lovers' lane. Soon, the hood of the car opens and closes and a man with a bag over his head with holes cut out for his eyes is seen holding wires he had yanked from the engine. While Sammy tries starting the car, the man breaks his window and pulls him out, cutting him on the broken glass. The man then gets inside the car with Linda.
The next morning, Linda is found on the side of the road barely alive. While at the crime scene, Deputy Norman Ramsey reports that both victims are still alive. He leaves a message for Sheriff Barker to meet him at Michael-Meagher Hospital. At the hospital, a doctor tells Sheriff Barker that Linda was not raped but that her back, stomach, and breasts were "heavily bitten; literally chewed." At the police station, Barker suggests to Police Chief Sullivan to warn teens and college students from parking on lonely roads.
On March 24, while investigating a lovers' lane in heavy rain, Ramsey hears gunshots and finds Howard W. Turner dead in a ditch and the corpse of his girlfriend, Emma Lou Cook, tied to a tree. Ramsey spots the hooded man escaping in a car. Panicked, the town sells out of guns and other home safety equipment. Sheriff Barker calls in help and tells Ramsey they are getting the most famous criminal investigator in the country, the "Lone Wolf" of the Texas Rangers, Captain J.D. Morales. After arriving, Morales explains he'll be in charge of the investigation and calls the unidentified attacker a Phantom. Ramsey is assigned to assist Morales, and Patrolman A.C. Benson "Sparkplug" is to be his driver.
At the barber shop, Ramsey explains to Morales his theory that the Phantom attacks every 21 days. The next attack falls on the day of a high school prom, and decoys are set up on the edges of town. After the dance, on April 14, trombone player Peggy Loomis leaves with her boyfriend Roy Allen. Despite her worries, they go to Spring Lake Park in the middle of town. When they leave, the Phantom jumps on the driver's door and pulls Roy out of the car, causing Peggy to wreck. She flees as the Phantom beats Roy, but he catches her and ties her hands around a tree. Roy awakens but is shot to death while attempting to escape. The Phantom attaches a pocket knife to Peggy's trombone and kills her while "playing" the instrument.
Morales and other officers meet with psychiatrist Dr. Kress at a restaurant where he explains that the Phantom is a highly intelligent sadist with a strong sex drive, between the ages of 35 and 40. As Kress expresses his doubts about their chances of capturing the Phantom, the Phantom's shoes are shown, revealing that he had heard the entire conversation. At the station, a man named Johnson says that he was robbed and forced to drive a man to Lufkin at gunpoint. While on the road, Ramsey receives a report about an armed suspect, and a brief chase ensues. The suspect, Eddie LeDoux, at first denies everything, then confesses to being the Phantom, but Morales is unconvinced. Johnson identifies him as his robber.
On May 3, Helen Reed is seen by the Phantom leaving a grocery store. At home that night, Helen asks her husband Floyd, who is sitting in front of a window in his armchair, if he hears somebody walking outside. After he replies that he does not, the Phantom shoots him through their window. Helen inspects and sees Floyd dying. As she uses the telephone to call police, the Phantom breaks through the screen door and shoots her twice in the face. Despite her wounds, she drags herself out of the house and into a cornfield while the Phantom inspects Floyd's body. The Phantom stalks her with a pickaxe, but leaves when she gets help at a nearby house. News of this attack causes the town to panic, and people begin boarding up their windows.
Later, Morales and Ramsey receive a report about a stolen car that matches the one from the Turner and Cook murders. While investigating a sand pit, they encounter the Phantom. Morales shoots at him but misses, causing him to run into the woods. The Phantom escapes by jumping past a moving train, but is shot in the leg. While waiting for the train to pass, the Phantom escapes. They continue their search but never find him. Years later, the film "The Town That Dreaded Sundown" premieres in Texarkana and the shoes of the Phantom are seen on someone standing in line.

Question: How often, according to Ramsey, does The Phantom attack?
Answer: Every 21 days.