This question came to me in the form of two questions but actually the answer to both is the same.
Here is the form the question took:
"1. In Zechariah 14:16-21, the Lord says He will smite those who do not come up to keep the feast of tabernacles. Now if we are not meant to be keeping the Holy Days, then isn't this a contradiction in the Bible?
2. In Isaiah 66:23, the Lord's second coming to the earth is prophesied and during this time, all flesh shall worship the Lord on the 'Sabbath'. Can you please explain to me, after the Lord has returned, how we would keep Sunday when he tells us that He will be worshipped on the Sabbath? (This appears to be another contradiction)."
First of all, I would briefly make the point that Christians should not have the attitude (which you infer) of "keeping Sunday" - The Lord's Day is not a sabbath day in that sense (although some areas of Christianity have tried to turn it into one), you tend to see it that way because of your former influence from seventh day observance.. The answer to both your questions is really the same, and in both cases, I would make two points. These points cover both the Scriptures/Questions which you raise:
1. The Double Inspiration of Scripture.
We need to understand what John Stott has called the 'double inspiration' of Scripture. God allowed the Old Testament prophets to write in their own styles and from within their own worldviews (just as, to a degree, He allowed that with the four Gospel writers); so the writings are (in that sense) partly inspired by those men, but - more importantly - inspired by God. But - again - God did not show them everything! To take Old Testament prophecy in general, obviously Christ is never specifically mentioned, but one of the prophets mentions Bethelehem, other prophets mentioned other things. Isaiah says quite a lot about Christ in chapter 53 - God inspired all this! But did the prophets understand exactly what they were writing or what events would occur at Bethlehem several centuries later? Very very unlikely! God only allowed them to see so much. One might compare this to straining one's eyes to see through a thick mist. This leads directly to the second point I would make, although, in a sense, I have already got into it:
2. Revelation is always progressive!
As you know, both Isaiah and Zechariah were Old Testament Prophets and operated from within their own scenario and worldview (as we have seen). The comment has often been made (and I am sure that you have heard it) that the Old Testament Prophets genuinely prophesied of the future but it is as though they saw these mountain peaks in the distance but were not able to pick out the detail in them, or the fact that the mountain peaks were not as close as they appeared to them. God allowed them to see just so much in their prophecies - but not everything! Even now God has only revealed so much to Christians - there are many aspects of the future which remain vague to us, God's time has not yet arrived to reveal more. There is little doubt that the prophets struggled with the full spiritual dimension of their prophecies. They wrote, but did not always fully understand. We err if we assume that they fully understood everything which they wrote. They often did not.
In the case of the Isaiah question, it is especially significant that the Sabbath is mentioned because the weekly Sabbath was a picture of that eternal Sabbath when all believers in perfect unison worship God in the eternal state to come. But did Isaiah understand what the writer of Hebrews 4 finally understood about the eternal, Christly dimension of the Sabbath? Almost certainly not! The writer of Hebrews 4 lifts up and actually spiritualizes the concept of the sabbath into an eternal 'sabbatism' or 'eternal rest' which Christians enter in Christ (see Matthew 11:28-30, and carefully consider the entire text from Hebrews 3:7 to 4:16). It is highly unlikely that Isaiah would have been able to cope with this concept from where he stood, under the Old Covenant, living somewhere around 750BC. He saw and Zechariah saw great glory and joy in the worship of God in the far distant future and their own worldview caused them to see such things in terms of the Sabbath and the Feast of Tabernacles. Revelation is progressive - so although God revealed certain things to the prophets He never revealed so much to them that they would then have been unable to continue functioning under the Old Covenant!
When saved and living in the distant future within the Eternal State, will we all attend a Feast of Tabernacles? I have no idea - maybe we will, but Christians do not choose to keep that festival today for the valid reasons of wishing to avoid legalism and any suggestion of creeping Galatianism (acknowledging Christ yet relying on the law). Paul shows us in several places that we do not need to feel under pressure in such matters (Galatians 4:8-11; Colossians 2:13-17; Romans 14:1-8).
I hope my comments help.
Robin A. Brace, 2006.
RECOVERING FROM ARMSTRONGISM