This is probably going to sound weird but here's what I believe:
I believe that every day is a dream, and when you wake up "the next morning" you're really just waking up from the dream that is yesterday into a 'layer' less of your dreams. I think you're already dead or at the very end and you're just living through every day, as the memories they are, in your dream(s). When you try to remember yesterday or any day in the past you recall it the same way as you recall something you experienced in your dreams, right? And you most likely don't remember absolutely everything that happened the previous day the same way you don't remember everything that happened in your dream.
Deja vu is the icing on the cake with my theory. At some point you feel like you've already experienced something and you understand it as deja vu because that's what you've been told it is and it's just "your brain messing up" or something like that. Thing is, at least in terms of this theory of mine, you have experienced it. It's one of the days you've experienced through your life- remember your body is already at the end and so has already experienced every day you've lived. You having "deja vu" in, say, 'day 1246' is your brain recalling another memory from a different day in your life.
A day is just a very lucid dream in my opinion, everything you see, do, hear, touch, etc is a projection of your mind. Other people are projections of your mind. The way they speak, move and do what they do is something your mind has created and you experience. There's probably flaws in my theory but whatever, that's what I believe haha.