What do we mean when we say it today?
What do Jewel, Lionel Richie, Kelly Rowland, Kelly Rowland, and The Offspring have in common? They all have a song called "Forever and a Day." If that doesn't show you how popular this quote is, we don't know what will. And that's just in the music industry. This one is everywhere you look.
Why? It can be applied to any situation, and it's so easy to understand. "Forever and a day" just talks about how something is taking forever (plus a little more). It might be something good: "Homecoming dance lasted forever and a day. It was an amazing night." Or something awful: "That test went on forever and day. I thought it was never going to end." Or anything in between.