Sometimes people aren't specific and just say "Spend in 2026" because that's when the statements have to close and the points have to hit.
What you for sure DON'T want to do is to mis-time your statement closing, hit the bonus with a statement close in December, and then have them hit the wrong year.
When I did my 2025-26 CPs, I called and had then change my closing date to very early in the month. That way, after I saw it close on 12/3/24, I had almost the whole month of December to spend knowing my next close was, for sure, 01/03/25 of the next year.