Today, this day - Vine's Expository Dictionary of New Testament Words
Today, this day[ 1,,G4594, semeron ]
an adverb (the Attic form is temeron), akin to hemera, a day, with the prefix t originally representing a pronoun. It is used frequently in Matthew, Luke and Acts; in the last it is always rendered this day;" also in Hebrews 1:5, and the RV of Hebrews 5:5 (AV, "to day") in the same quotation; "today" in Hebrews 3:7, Hebrews 3:13, Hebrews 3:15; Hebrews 4:7 (twice); Hebrews 13:8; also James 4:13.
The clause containing semeron is sometimes introduced by the conjunction hoti, "that," e.g., Mark 14:30; Luke 4:21; Luke 19:9; sometimes without the conjunction, e.g., Luke 22:34; Luke 23:43, where "today" is to be attached to the next statement, "shalt thou be with Me;" there are no grammatical reasons for the insistence that the connection must be with the statement "Verily I say unto thee," nor is such an idea necessitated by examples from either the Sept. or the NT; the connection given in the AV and RV is right.
In Romans 11:8; 2 Corinthians 3:14-15, the lit. rendering is "unto the today day," the emphasis being brought out by the RV, "unto (until) this very day."
In Hebrews 4:7, the "today" of Psalms 95:7 is evidently designed to extend to the present period of the Christian faith.