The idea behind the word today in Hebrews 3:13 and 15 is “while you still have time” or “while it’s not too late.” In other words now. Don’t put off encouraging your Christian brother or sister. Call them now. Visit them now. Don’t put off witnessing for Christ do it today while its not too late. It’s perfectly fine with the Devil if you wait until tomorrow just don’t do it today – while there is still time!
“To-morrow is the day when idle men work, and fools repent. To-morrow is Satan’s to-day; he cares not what good resolutions you form, if only you fix them for to-morrow.” (Quote taken from the Commentary Critical and Explanatory on the Whole Bible)