The key scriptures that we use to prove that gentile Christians were really keeping the Feast of Unleavened Bread with unleavened bread are in 1 Corinthians 5:6-8. Verse 6, Your glorying is not good; know ye not that a little leaven leaveneth the whole lump? Verse 7 states, Purge out therefore the old leaven that ye may be a new lump, AS YOU ARE UNLEAVENED. For even Christ our Passover is sacrificed for us: verse 8, 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.
Paul is saying to purge out (get rid of) the old leaven (sin), that ye may be a new (unleavened) lump (spiritually) as ye are (actually) unleavened. He wrote this to them during the Days of Unleavened Bread.
Those who don't want to keep the Days of Unleavened Bread twist these scriptures and try to say that gentile Christians were not physically keeping the Days of Unleavened Bread. They say "as you are unleavened" does not mean what it says, but is a simile. A simile tries to make a point by comparing two things that are not alike, for instance, a man and a turtle. "He is as slow as a turtle is a simile. A simile uses "like or as" for this comparison. In our simile, he and turtle are not alike, but the turtle is used to point out that the man is slow physically. If 'as ye are unleavened' is only meant by Paul as a simile and not to mean that they were actually unleavened, then these scriptures would by saying Be free from leaven (sin) metaphorically), as you are free from leaven (sin) again metaphorically."
Similes are not constructed this way. It is illogical. Remember that a simile is a comparison between two things that are unlike. In the case of our scripture, the spiritual is being compared with the physical. Otherwise the simile makes no sense because it would not be comparing two things that are different. What Paul must be writing is "Be free from leaven" (sin) spiritually, as you are free from leaven literally. (Physically) It would be like saying be as fast as a turtle when a turtle is clearly not fast. Be unleavened spiritually as you are not unleavened physically makes no sense.
The Greek gentile Christians were literally unleavened years after Christ's death, as Acts 20:6 and 12:3 plainly show. Paul tells these gentiles to keep the Feast. He is clearly speaking of the Feast of Unleavened Bread and the New Testament Passover or Lord's Supper.
For timing issues look at the phrase I Cor11:23 'the same night in which he was betrayed'. It was not the evening of the Jewish Passover.