29 April 711 - Islamic conquest of Hispania: Moorish troops led by Tariq ibn-Ziyad land at Gibraltar to begin their invasion of the Iberian Peninsula (Al-Andalus).
28 April 1192 - Assassination of Conrad of Montferrat (Conrad I), King of Jerusalem, in Tyre, two days after his title to the throne is confirmed by election. The killing is carried out by Hashshashin.
Read about this and the events surrounding it in Richard Warren Field's The Swords of Faith.
More llike it at medieval-novels.com with over 1500 novels and movies set in the Middle Ages.
27 April 1509 - Pope Julius II places the Italian state of Venice under interdict. As i said the other day, that ruined the day for hundreds of thousands of people!