Lady Macbeth didn't "know" about Banquo's murder. In other words, Macbeth planned the murder and refused to tell Lady Macbeth about it. In some productions of Macbeth, the mysterious third murderer who kills Banquo is Lady Macbeth. Thus, if you believe this to be true, you can write a Lady Macbeth dairy entry talking about snooping around, finding out about the plot to kill Banquo, dressing up as the third murderer, and going to kill Banquo.