Hebrews 4:7 tells us: “He again fixes a certain day, ‘Today,’ saying through David after so long a time just as has been said before, ‘Today if you hear His voice, do not harden your hearts’”.
God has fixed a “certain day” in which all who hear His word should heed it. That day is “today”! In other words, in whatever day you hear His word, you had best respond to it then. It is never “tomorrow” with God – though that is the response many of us like to give Him. We often offer our obedience “tomorrow”:

— when we start a new year … or a new week … or a new pay period — and so on, and so on …
— when the kids are grown
— when I get a better job
— when this present crisis is over
— when things settle down (as if that will ever happen!) etc.

God knows very well that the obedience we would promise “tomorrow” never comes. If you are going to obey Him, it is going to be today. That is why He says in II Corinthians 6:2, “NOW is the acceptable time; NOW is the day of salvation.” TODAY is the only day you know for certain you actually have. That is why God tells us here in Hebrews that He has fixed a certain day in which you are to obey that command He has given you – and that day is: “TODAY”!