Roger Mortimer married Jeanne de Geneville on September 30, 1301, the eve of the Feast of St. Matthew the Apostle. The groom was 14, and the bride was 15. Ian Mortimer, in his biography The Greatest Traitor, notes that a comet appeared the night after the wedding, and was visible for a week. This would appear to be a cited appearance of Haley's Comet, which did indeed appear in 1301 on it's 75-year-cycle around the earth. That year Italian painter Giotto di Bondone included the comet in the place of the Star of Bethlehem in a painting of the Nativity. Scroll down on this article to see an image of this painting featuring Haley's Comet as Roger would've seen it.
Here are the years that Haley's Comet would've appearedv during the medieval era: