Æbelholt Abbey (Danish: Æbelholt Kloster) was an Augustinian monastery situated in Tjæreby, 5 km west from Hillerød.
The monastery was first established on Eskilsø Island in Roskilde Fjord in 1104. While no remains of the wooden abbey have survived, the stone abbey church there still remains, though in ruins. It was 24 meters long and had a nave, choir, and apse in the Romanesque style. The monks became "unruly" and Bishop Absalon of Roskilde, determined to obtain a new Augustinian superior, sent for his friend, William, abbot of the abbey of Sainte-Geneviève in Paris. When William arrived in 1165 with three French canons there were only six religious left at Eskilsø, two of whom were dismissed when they refused to submit to the new rule.
In 1167 the abbey moved to Æbelholt in Tjæreby, supported by a donation of land from Absalon in Tjæreby Parish and endowed with several income-producing farms, tithes from many north Zealand churches, and several mills. The monastery on Eskilsø was closed. The first church and abbey at Æbelholt were made of timber. Construction began almost immediately on a new abbey church of limestone which was completed in 1210.
Abbot William experienced considerable initial difficulties. The three French canons returned to Paris, finding conditions in Denmark too bleak. A few of the Danish canons plotted to murder him when he ordered that they eat "herbs and leaves" instead of their usual meals. The "Life of St. William" notes some of the ways in which they considered killing him, either by setting fire to a pile of straw near his bed in the dormitory, putting him in a sack and drowning him, or taking an axe to him. Eventually William's piety, fairness, wisdom, and intelligence won them and the local populace over. He was considered a saint in his own lifetime. He was highly regarded by several kings of Denmark and served as an intermediary between the pope and the Danish monarchy.
Abbot William died on 5 April 1203 at the age of 75. Miracles at his grave and in connection with his relics brought pilgrims in great numbers, and the abbey developed into the greatest Augustinian house in the north. By 1210 the list of miracles and signs recorded was so great that the Archbishop of Lund, Anders Sunesen, petitioned Pope Honorius III for his canonization. In 1219 the pope authorized several bishops to investigate the claims with an eye to making William a saint. The result was a book The Life of Abbot William which was submitted for the pope's consideration.
William was canonized in 1224. On 16 June 1238 with great ceremony William's body was translated to lie inside the high altar in the new abbey church. A small separate chapel was constructed over the his previous grave, so pilgrims could visit without disturbing the monks. In time relics of Saint William were given to Roskilde Cathedral, Lund Cathedral, the Church of Our Lady (Vor Frue Kirke) and Greyfriars Church in Copenhagen, and Greyfriars Church in Roskilde.