Today is the day, not tomorrow. Not next week. Not later.

Hebrews 3:7-10 (ESV)
7 Therefore, as the Holy Spirit says,  “Today, if you hear his voice,
8 do not harden your hearts as in the rebellion, on the day of testing in the wilderness,
9 where your fathers put me to the test and saw my works
10 for  forty years. Therefore I was provoked with that generation, and said, ‘They always go astray in their heart; they have not known my ways.’

Hebrews 3:15 (ESV) As it is said,  “Today, if you hear his voice, do not harden your hearts as in the rebellion.”

Hebrews 4:3-7 (ESV)
3 For we who have believed enter that rest, as he has said,  “As I swore in my wrath, ‘They shall not enter my rest,’” although his works were finished from the foundation of the world.
4 For he has somewhere spoken of the seventh day in this way:  “And God rested on the seventh day from all his works.”
5 And again in this passage he said,  “They shall not enter my rest.”
6 Since therefore it remains for some to enter it, and those who formerly received the good news  failed to enter because of disobedience,
7 again he appoints a certain day, “Today,” saying through David so long afterward, in the words already quoted,  “Today, if you hear his voice, do not harden your hearts.”