1 July 1916: The Somme

Comprising the main Allied attack on the Western Front
during 1916, the Battle of the Somme is famous chiefly
on account of the loss of 58,000 British troops (one third
of them killed) on the first day of the battle, 1 July
1916, which to this day remains a one-day record.The
attack was launched upon a 30 kilometer front, from
north of the Somme river between Arras and Albert,
and ran from 1 July until 18 November, at which point
it was called off.

http://www.firstworldwar.com/battles/somme.htm