Q: Didn't God say that His feasts were to be observed forever?
A: Who did He say that to? "And the Lord spake unto Moses, saying, Speak unto the children of Israel, and say unto them, concerning the feasts of the Lord, which ye shall proclaim to be holy convocations, even these are my feasts."
In other words, the instructions about the feasts were given to the children of Israel, not to the world at large.
Q: Didn't Zechariah predict that everyone would observe the feasts in God's Kingdom?
A: Yes, at Jerusalem. "And it shall come to pass, that every one that is left of all the nations which came against Jerusalem shall even go up from year to year to worship the King, the Lord of hosts, and to keep the feast of tabernacles. And it shall be, that whoso will not come up of all the families of the earth unto Jerusalem to worship the King, the Lord of hosts, even upon them shall be no rain." (Zechariah 14:16-17
Q: Doesn't forever mean forever? Didn't God intend for the feasts to be observed for all times?
A: The Hebrew word "owlam" can certainly indicate eternity in some contexts, but it very often is used to indicate that something is perpetual in nature. God promised that David's dynasty would be eternal. Does the fact that David's dynasty ceased to rule for a time make God's promise to him void? Does the fact that the Israelites ceased to observe the feasts for many years after the destruction of both temples somehow nullify Zechariah's prophecy about their future observance?
Q: Didn't Paul tell Christians at Corinth to keep the Feast of Unleavened Bread?
A: Paul was correcting the Corinthian Church for their acceptance of sexual immorality in their midst. Specifically, one of their members was openly carrying on an affair with his father's wife. (I Corinthians 5:1) He tells them that they should be ashamed of this, but they are puffed up (prideful) about their tolerance of the situation. (I Corinthians 5:2) Paul then proceeded to command them to put the individual responsible for this sinful behavior out of the church! (I Corinthians 5:3-5) In this connection, he uses the Feast of Unleavened Bread as a metaphor to explain the negative impact that this is having on the entire congregation. He wrote: "Your glorying is not good. Know ye not that a little leaven leaventh the whole lump? Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened. For even Christ our passover is sacrificed for us: Therefore let us keep the feast, not with old leaven, neither with the leaven of malice and wickedness; but with the unleavened bread of sincerity and truth." (I Corinthians 5:6-8) Once again, Paul is calling upon the spiritual principles upon which the feast was based. Hence, this Scripture has absolutely nothing to say about whether or not the Corinthians were observing the Feast of Unleavened Bread.
Q: Didn't Christ imply that it would be ok to designate feast sites other than Jerusalem when he said, "For where two or three are gathered together in my name, there am I in the midst of them?" (Matthew 18:20)
A: If that Scripture can be used as a justification for multiple feast sites, then logic and fairness dictate that it be used as a justification for Christians gathering together on Sunday!