This event is remembered by Heavy Rockers and Metallifans all over the world, and is tributed on the 26th September every year, especially milestone years. (All information thanks to Wikipedia)
To Live is to Die
It is a tribute to their former bassist Cliff Burton who was killed in a bus crash in 1986. Burton actually wrote the riffs for the song a few months before he died. Though the track is considered an instrumental track, it contains a few lyrics near the end of the song written by Burton which are spoken by James Hetfield.
When a man lies he murders some part of the world
These are the pale deaths which men miscall their lives
All this I cannot bear to witness any longer
Cannot the kingdom of salvation take me home