Yesterday is gone with all that it contained. ذَهَبَ أَمْسِ بِمَا فِيهِ‏.

The first to say this was Damdam ibn Amr al-Yarbu'i. He was in love with a woman and pursued her with every trick, but she refused him. Ghur ibn Tha'labah ibn Yarbu' used to visit her. Damdam followed them and found them together in one place. He sat in a wine cellar beside them, seeing them but not being seen by them. Ghur said: "Once she came to me, yet she herself refused me, / For the man, the wanderer of the hill, Damdam." Damdam attacked him and killed him, saying: "You will know that I am not safe from my enemies, / And that you will be far from her if you distance yourself." He was asked: "Why did you kill your cousin?" He replied: "Yesterday is gone with all that it contained." And his saying became a proverb.
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The proverb "Yesterday is gone with all that it contained" is used to describe something that has passed and is finished, so there's no point in going back to it or clinging to it. Its meaning is: what happened yesterday is gone, with all its good or bad, and cannot be changed now.
The closest English proverbs and expressions to it are:
"Let bygones be bygones."
This is the most common and closest equivalent.
"What's done is done."
"Yesterday is gone."
"The past is the past."
"Don't cry over spilled milk."
This means not to dwell on the past.
The best equivalent to the Arabic proverb is:
"Let bygones be bygones."
Or:
"What's done is done."
Both convey the meaning: the matter is over and there is no point in digging up what has passed or talking about it again.
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‏أول من قال ذلك ضَمْضَم بن عمرو اليَرْبُوعي، وكان هَوِىَ امرأةً، فطلبها بكل حيلة، فأبت عليه، وقد كان غر بن ثعلبة ابن يربوع يختلف إليها، فاتبع ضمضمٌ أثَرَهما وقد اجتمعا في مكان واحد فصار في خَمَر إلى جانبهما يراهما ولا يريانه، فقال غر‏:‏
قديماً تُوَاتِيِني وتأبى بنفسها * على المرء جَوّاب التَّنُوفَةِ ضَمْضَمِ
فشد عليه ضمضم فقتله، وقال‏:‏
ستعلم أني لست آمن مبغضا * وأنَّكَ عَنْهَا إن نأيْتَ بمَعْزِلِ
فقيل له‏:‏ لِمَ قتلت ابن عمك‏؟‏ قال‏:‏ ذهب أمس بما فيه، فذهب قولُه مثلاً