I had posted the following in another thread:
LUD is "Last Update Date". After joining this forum i did not know how to see it for a long time. But recently I figured it out from an old post. If you go to USCIS status check website and type your case number you will simply see the status message of your case but not the LUD. To see the LUD you have to create a Portfolio account on the USCIS website. Once you are registered, then add your case (using the case number) to the portfolio. Then when you login you will see a table showing all cases that you have added to your portfolio. This table has a column called Last Update, which has this mysterious last date USCIS updated some info on that case. Sometimes you see these dates changing, but your case status message stays same and then you get people posting here "LUD changes ... don't know what's happening" etc. But those magical LUDs sometimes make our days while waiting in line to see the end of the tunnel for years with hair going gray or falling out. It's a little glimmer of hope to convince ourselves "ah, they are doing something!".