One of the most famous instances of one army attacking another on a holiday was when George Washington crossed the Delaware on Christmas Day:  “During the night of December 25, Washington led his troops across the ice-swollen Delaware about 9 miles north of Trenton. The weather was horrendous and the river treacherous. Raging winds combined with snow, sleet and rain to produce almost impossible conditions. To add to the difficulties, a significant number of Washington’s force marched through the snow without shoes.  The next morning they attacked to the south, taking the Hessian garrison by surprise and over-running the town. After fierce fighting, and the loss of their commander, the Hessians surrendered.”

http://www.eyewitnesstohistory.com/washingtondelaware.htm

This attack gave Washington a much needed victory and gave his troops a reason to reenlist for 1777.

Five hundred years earlier, on the morning of 22 March 1282, Palm Sunday, Welsh insurgents attacked the English castles at Hawarden,  Flint, Rhuddlan, and the borough of Oswestry.  Three days later, on the Feast of St. Mary’s, Aberystwythe Castle fell.  J. Beverley Smith writes:  “Gruffydd ap Maredudd, acting in association with Rhys Fychan ap Rhys ap Maelgwn came to the constable and invited him to dinner.  The constable could hardly have been aware of the happenings in the north-east [of the country] and accepted the invitation, only to be taken captive by his host while a force of Gruffydd’s men took possession of the castle” (Llywelyn ap Gruffydd, pg. 452).

The desperation that drove the Welsh to attack during the Christian Holy Week was similar in many ways to what drove George Washington to violate the peace of Christmas.  Unfortunately, for the Welsh, Edward I took their violation as evidence of their barbarism and his divine right to rule them.

Other notable Holy Day attacks include:

6 October 1973–Egypt and Syria attack Israel on Yom Kippur, the Day of Atonement, the holiest day in the Jewish calendar.

7 December 1944 (8 December in Japan)–Attack on Pearl Harbor by Japanese forces.  This was not a holy day to Americans, but it was to the Japanese, as the day on which Shakyamuni realised the Way.  For this reason, it had been a day for commemorating the liberation of humankind in Japan.  http://vishvapaniswriting.blogspot.com/2007/02/rude-awakenings-zen-at-war.html

31 May 1918–German long-range bombers attacked Cologne, France on Corpus Christi Day, which is/was a very, holy day in the Roman Catholic Church, prompting outrage on both sides of the Atlantic.

http://query.nytimes.com/mem/archive-free/pdf?_r=1&res=9D03E2DB163EE433A25752C0A9609C946996D6CF

It has become fairly common in recent years for terrorist attacks on both Islamic and Christian Holy Days.