One of the bloodiest battles in the history of mankind, the Battle of the Somme took place over 141 days and the British Army lost 19,241 soldiers on just the first day of fighting. Ultimately, both sides lost over 1.5 million men in what must truly have been one of the most horrific experiences a person could ever experience.
A timeline of events can be found at The BBC, and The History Channel has a high level overview, as well, should you want to learn more and need a place to start.