This quotation in Shakespeare's Julius Caesar, delivered by Titinius, is spoken in response to Cassius' death:
No, this was he, Messala,
But Cassius is no more. O setting sun,
As in thy red rays thou dost sink to night,
So in his red blood Cassius' day is set,
The sun of Rome is set! Our day is gone... (V.iii.63-67)
Messala sees Cassius' body on the ground and asks the devastated Titinius to confirm that it is, indeed, Cassius that he sees. Titinius notes that it is the body of the man, but that Cassius is gone. The metaphor is introduced, then, to compare Cassius' death to the sunset (the death of a day). As the day dies with a blazing red sunset, Cassius also dies, bathed in red—his life's blood, and his "day" (his life) is over. Titinius notes that their hopes for Rome (more of the extended metaphor with "sun") are gone. Their joy of Rome is past—the best of their days are behind them.
Titinius is Cassius' best friend. In the last line of this speech, he notes that in Cassius' passing, the glory of their days in Rome is gone. There could be a play on words here (a pun) with "sun" and "son"— meaning that this child ("son") of the Roman Empire that would have seen it to greatness is now dead ("set"). In his friend's death, Titinius sees that all is lost.
When Titinius returns and sees his friend slain, he mourns the end of Rome as he has known it
Regardless of whether Brutus is successful against Octavius, Titinius sees nothing left to fight for. The reader gets the sense that for Titinius, Cassius was more than enough reason for him to fight for Rome. Titinius is truly dedicated to Cassius—perhaps even more than to Cassius' plans. In discovering Cassius dead, Titinius' purpose in life has died as well.