Precious little details have survived of Saint Denis's early life but what we do know comes from Saint Gregory of Tours's 6th century Historia Francorum. Saint Denis was sent from Italy under the direction of Pope Fabian to aid in the conversion of peoples living in the Roman area known as Gaul. Saint Denis was part of the so called "Holy Helpers".
We know he travelled to Paris (Then called Lutetia or Lutece), the home of the Parisii. The Parisii were a Gaulish tribe living on the banks of the Seine river and were known to be polytheists. The process of conversion was so successful that in a very short amount of time the local non-Christian priests began complaining to the Roman government. Their words fell on sympathetic ears as the Diocletian persecutions were rapidly approaching.
At first, the now Bishop Saint Denis and his followers set their place of worship in a crypt underground. That crypt has survived through the ages (though the original church built upon it was dismantled during the French Revolution) and can be found still under a parking garage at number 14bis Rue Pierre Nicole in the 5th Arrondissement. Their hiding place did not last long and under the leadership of the Emperor Valerian, Saint Denis and his companions were arrested.
After several years in prison, the order of execution came. Saint Denis and two of his clergy were executed by beheading on the highest hill of the town (Now referred to as Montmarte - the Mountain of Martyrs so called due to the large number of Christians killed there). Although legend holds that after his execution Saint Denis carried his head a few kilometers to the place of burial he chose, we do know for certain that Saint Denis was buried North of Paris in a town now called Saint-Denis.
This burial place quickly became extremely popular. An original simple shrine erected shortly after his burial became a small church which turned into an abbey. King Dagobert I in the 7th century choose this abbey his burial place. This event turned into a tradition, and since then French Kings have been buried in the church of Saint-Denis. Visitors can tour the Saint Denis Basilica now, but very little of the stonework remains from the original building.
Saint Denis is celebrated as one of the fourteen Holy Helpers, specifically for intercession against headaches and for exorcisms. His feast day is celebrated on October 9th (having been placed into the Roman Calendar by Pope Pius V in 1568) and linked to folklore regarding the upcoming winter weather in France. A traditional saying in France is
“A la Saint-Denis, l’Hiver fait son nid.”
“The winter settles on Saint Denis’s day”.