Proverbs 27:1 warns against boasting about what, since you don't know what a day may bring?

Answer: Tomorrow

"Do not boast about tomorrow, for you do not know what a day may bring." The next verse adds: let another praise you, not your own mouth.

“Do not boast about tomorrow, for you do not know what a day may bring.”— Proverbs 27:1 (BSB)