The treasury bill rates is announced (typically) on Mondays, AFTER the auction — when it is too late to buy them at that rate!
Next week the auction will take place on Tuesday, because of the Labor Day holiday on Monday.
You open your account and put in your request — and settle for the rate that is set by the big banks at the auction. YOu get the “average” rate — the rate that is announced after the auction.
Read this for details: https://www.terrysavage.com/t-bills-beat-cds/