What should I do the day before submitting my PhD thesis?
I would suggest reading through it one more time for pleasure. Savor the writing you labored so hard on, marvel at just how much work has gone into this, and reflect on where you are now compared to where you were when you started this labor.
To abuse the poetry of Genesis:

And the student saw every thing that they had made, and, behold, it was very good.
Thus the thesis was finished, and all the host of its data. And on the seventh day the student ended their work which they had made.