This is the second feast of Israel and the second of days in God's 7 day plan, remember the verse that says "a day is a thousand years , and a thousand years a day'.
Jesus fulfilled Passover and then comes this feast.

After the Passover lambs were slain at twilight the Feast of Unleavened Bread began at sunset. All leaven, a type of sin, was removed from the houses and the unleavened bread was eaten throughout the seven days of the feast.		As the 15th day of Nisan began at sunset after Passover Jesus was buried. He was the sinless unleavened 'Bread of Heaven' offered as the perfect sacrifice. Thus He fulfilled the Feast of Unleavened Bread.

The scripture stipulates the duration of the feast. It is to last seven days. The first day, which is on the 15th of Nisan and the day following Passover Eve is day #1 of the feast. This first day is a sabbath, a sacred assembly, a holy convocation or a high holy day. The seventh day is also a sabbath.

Leaven in the scriptures symbolizes sin.
From the Christian perspective, the picture is quite clear.
Jesus was the unleavened sinless Bread of Heaven.
His life was one of sinless perfection.
And His body was offered up to YHVH/God.
He was the acceptable and perfect sacrifice for sin.

The Feast of Unleavened Bread is also a calling to God's consecrated people.
They are exhorted to journey on with their Saviour in a walk of holiness.
In Christ we partake of the unleavened Bread.
Through Him we enter into a new state of being.
We come into submission to YHVH/God in unpretentious sincerity and truth.
Here are the words of the Apostle Paul on the matter.

1 Corinthians 5
7. Therefore purge out the old leaven,
that you may be a new lump,
since you truly are unleavened.
For indeed Christ, our Passover,
was sacrificed for us.
8. Therefore let us keep the feast,
not with old leaven,
nor with the leaven of malice and wickedness,
but with the unleavened bread of sincerity and truth.

When the Israelites left Egypt they left in haste. The bread they took with them was unleavened bread, baked in haste for a people who were on the move and preparing their victuals to travel. The Feast of Unleavened Bread commemorates the time when the Israelites left Egypt. Their next waypoint was the crossing at the Red Sea. They bread they took with them was not puffed up. It was baked without yeast. There simply was not time enough to allow the bread to rise.

THE BREAD FROM HEAVEN

The Bread from Heaven was seen in the earlier history of Israsel.
And the heavenly food of provision was spoken of by the prophets.
The Bread of Heaven is seen repeatedly in the poetry of the Old Covenant.

Nehemiah 9:15 NKJV
You gave them bread from heaven for their hunger,
And brought them water out of the rock for their thirst,
And told them to go in to possess the land
Which You had sworn to give them.

Psalm 78:24
Had rained down manna on them to eat,
And given them of the bread of heaven.

Psalm 105:40
The people asked, and He brought quail,
And satisfied them with the bread of heaven.

THE NEW COVENANT FULFILMENT OF THE FEAST OF UNLEAVENED BREAD

The New Covenant fulfillment of the Feast of Unleavened Bread was seen in the Passover of Christ's passion. Jesus was buried in the earth just in time for the Feast of Unleavened Bread. Here below is a calendar of that epic year of Christ's passion. It lays out the seven days of the Feast of Unleavened Bread. It also shows the back to back sabbaths that precede the Feast of Firstfruits. Firstfruits is day #1 of the fifty days leading up to the Feast of Pentecost.