Luke 4:19 should be treated rather delicately because of these observations:
- The two phrases that it alludes to in Isa 61:2 are in quintessential Hebrew Parallelism: "acceptable YEAR of the LORD" vs "DAY of vengeance of our God". This is typical of many other places in the OT that places in "Day" and "Year" in parallel forms and (in the absence of numerical adjectives) means a period of time without necessarily being too specific. See for example: Isa 34:8, 61:2, 63:4, Job 10:5, 15:20, 36:11, Ps 39:5, 61:6, 90:10, 15, Eze 22:4. (In other places the Hebrew has "day" or "days" when clearly "year" or "years" is intended: Gen 5, Ex 13:10, 1 Sam 2:19, 20:6, 27:7.)
- Jesus also uses this classic Hebrew idiom when we observe that He says "Year of the Lord's favour" and then asserts (v21), "this day, this scripture has been fulfilled". Hence, Jesus believed that the "year of the Lord's favour" had arrived as He was preaching. That is, Jesus used the quote from Isaiah as the keynote message of His earthly ministry and Gospel.
- Jesus, when he quoted Isaiah, He did NOT quote "the day of vengeance of our God". That is, Jesus specifically chose what He wanted from Isaiah's prophecy (which contains much else) and repurposed it for His own.
The idea of "the acceptable year" or, "year of the LORD's favour" is clearly an allusion by Isaiah to the Jubilee, as Ellicott has observed.
The acceptable year of the Lord.--The primary reference was to the
year of Jubilee, when land that had been mortgaged returned to its
owner, and debts were forgiven, and Israelite slaves released
(Leviticus 25:9-10). It was to our Lord, as it had been to Isaiah, the
type of the "year" of the divine kingdom. A somewhat slavish
literalism, which the study of St. John's Gospel (Luke 2, 5, 6, 7, 12)
would have dispelled in an hour, led some of the Fathers to infer from
this that our Lord's ministry lasted but for a single year.
Thus, the "year of the Lord's favour" is not a literal year but the time when Jesus began His ministry and the proclamation of the Gospel when lasts until now.