(Pro 27:1 KJV) Boast not thyself of to morrow; for thou knowest not what a day may bring forth.
Is there anything that we are to “boast” (1984b) about? We are surrounded by people boasting constantly. Politically we hear the bully pulpit and, in this election year, we hear advertisements. I was once told after a poor job interview that I should tell people my strong points and not my weaknesses. Paul boasted in his weakness.
(2Co 12:9 KJV) And he said unto me, My grace is sufficient for thee: for my strength is made perfect in weakness. Most gladly therefore will I rather glory in my infirmities, that the power of Christ may rest upon me.
I think this may be a matter of contrast. Our best is very little compared to what God offers. Maybe we should spend our time praising God instead of reminding people of what we have done.
James also had something to say about this in terms of “tomorrow” (4279)
(Jas 4:13-14 KJV) Go to now, ye that say, To day or to morrow we will go into such a city, and continue there a year, and buy and sell, and get gain: Whereas ye know not what shall be on the morrow. For what is your life? It is even a vapour, that appeareth for a little time, and then vanisheth away.
The Bible does tell us to plan. We are to count the cost. But we are to do it with the understanding that God may have things in store we could never dream of. Jesus also spoke about making assumptions. We go forward with confidence because we trust in God, not because we are great planners.