Today is the only day. This means we shouldn’t fret about tomorrow:
“Give your entire attention to what God is doing right now, and don’t get worked up about what may or may not happen tomorrow. God will help you deal with whatever hard things come up when the time comes.” (Matthew 6:34 The Message)
And it means that we need to let go of the past:
But one thing I do: Forgetting what is behind and straining toward what is ahead, I press on toward the goal to win the prize for which God has called me heavenward in Christ Jesus. (Philippians 3:13-14 NIV)
Today is the only day.