This one Trawsfynydd (scroll down) says December 15th. And I found another site that said his day was Dec. 9th.

St. JOHN ROBERTS                                    

One of the 40 Martyrs of England and Wales who was canonized in 1970. He was born at Rhiwgoch, Trawsfynydd in 1577, and was probably baptized at the St. Madryn's church in the village. It is mentioned that his family lived at either Gelli Goch or Tyddyn Gwladus. He was reared a Protestant and on the 26th February 1595/6 enlisted in St. Johns College, Oxford. He then travelled the continent and whilst in Paris turned Catholic.

In 1599 he went to St. Benedict Abbey, Valladolid; he then went on to novitate at Santiago de Compostela, about the end of 1600. He joined the Benedictine faith and adopted then name Fray Juan de Mervinia, brother John from Merioneth. He was ordained and went out on the English mission on the 26th December 1602 - he was the first monk to return to England after Henry VIII closed the monasteries. He was four or five times detained by the authorities, but on each occasion following a short term in prison he was deported. During this period he made a name for himself in London for helping people who were suffering from the plague. He again returned but was arrested and convicted of high treason. He was executed by being hung, drawn and quartered at Tyburn, London on the 10 December 1610.
St John was blessed on the 15th December 1929, and canonized on the 25th of October 1970.