Do not boast about tomorrow,
for you do not know what a day may bring forth.
This is an unchanging truth. No matter how hard you try, "...you do not know what a day may bring forth." It might be easy, it might be difficult, it might be the hardest, saddest day of your life... or it might be the most exciting day you have ever had. There is just no way of knowing.
So don't even try. Don't brag about or boast about your control over tomorrow, because you don't have control over it.