אַֽל־תִּ֭תְהַלֵּל בְּי֣וֹם מָחָ֑ר כִּ֤י לֹא־תֵ֝דַ֗ע מַה־יֵּ֥לֶד יֽוֹם׃
Boast not thyself of to- morrow; For thou knowest not what a day may bring forth.
Rashi on Proverbs
Do not boast Do not boast now concerning a thing that is destined to be tomorrow.
Rashi on Proverbs
for you do not know what the day will bear Perhaps the day will bear some evil that will nullify it.