The phrase 'Let Bygones Be Bygones' is used to indicate that unpleasant things that have happened in the past are best forgotten.
Example of Use: “Now that we’re friends, we can let bygones be bygones.”
The phrase 'Let bygones be bygones' originated in the 15th century. A good example was recorded in a letter by Scottish churchman Samuel Rutherford, acknowledging the follies of his youth: “Pray that byegones betwixt me and my Lord may be byegones.”