These booklets explain the significance of God's holy days in far more detail than I can:
The following quotations are from something I wrote more than a decade ago:
A Very Short History of the World — a Biblical Christian View
It's perhaps an interesting coincidence that God's 7 festivals occupy 19 days, while the sacred calendar requires the addition of a leap month 7 times in every 19 year cycle.
The Spring feasts:
Passover (Lev 23:5)
The final plague was the death of the firstborn son of each family
To escape this plague themselves, the Israelites were told to mark their doors with lamb's blood
Their children were saved, but the Egyptians were not so fortunate, including the Pharaoh himself, who relented and allowed the Israelites to leave.
The event has been celebrated as Passover each year thereafter on the 14th of Nissan as a reminder of how their firstborn sons had been spared from death, just as Abraham and Sarah's firstborn son Isaac had been saved by the blood of a lamb
The evening meal included mutton, which was to be consumed without leftovers and without breaking any bones.
On the Sunday following Passover, a priest would wave a sheaf of barley towards the heavens as an offering to God from the first-fruits of the early spring harvest
Much of the significance of this lamb and first-fruits symbolism wasn't obvious at the time.
The shed blood to pay for sins, the lamb to replace Isaac, the sacrifice of a first-born son, all represented Jesus's death as a payment for mankind's sins.
Each year as we celebrate Passover, we accept his broken body and shed blood in the symbolic form of bread and wine, and in doing so accept Christ's sacrifice as payment for our past sins.
Unleavened Bread (Lev 23:6-8) and Firstfruits (Lev 23:11)
After leaving, the Israelites were pursued by the Egyptians but were led by God to safety after crossing into the Arabian desert.
During this flight, food had to be prepared quickly so there was no time to leaven bread, which had to be made from flour and water without any yeast.
Each year, the week after Passover is celebrated by avoiding all forms of yeast as a reminder that Israel had escaped from slavery in Egypt.
Again it wasn't obvious at the time, but the exodus from Egypt was symbolic of how God's people had to separate themselves from the sins of the world, both removing themselves from sinful influences and removing and keeping sin out of their own lives.
Like yeast, even a small amount of sin can grow and take over, so avoiding yeast for one week each year became a symbolic reminder of how one must always be on guard against allowing even seemingly trivial sin to infect one's life.
The exodus from Egypt and the Days of Unleavened Bread represent how we not only must remove ourselves spiritually from the physical world of sin, but must reject all sources of sin from our lives and be continuously on guard against any infiltration.
Pentecost (Lev 23:15-16)
Fifty days after leaving Egypt, the Israelites received a written copy of the ten commandments directly from God.
These commandments had always been in effect and known by mankind (e.g. Abraham was noted as having followed them), but now they had been set down as part of a permanent written record.
Then, and during the ensuing years, Moses recorded many of God's other existing laws (e.g. Abraham tithed, Noah knew about clean and unclean meat), and codified appropriate practices for honouring the weekly sabbath and the seven annual holy celebrations (Passover, Unleavened Bread, and Pentecost in the Spring, and four others in the Autumn).
These writings comprise the first five books of the Bible and are known as the Torah, or the Law.
Each year, this Feast of Weeks (a week of weeks is 49 days) was to be celebrated to commemorate the receiving of God's laws.
It was also known as the Feast of Harvest, when the first-fruits of the late spring harvest were gathered and offered as thanksgiving to God.
Pentecost was when God ratified his Covenant with the people of Israel, who were to be the physical first-fruits of his own Harvest.
The symbolic significance wouldn't become apparent until 1300 years later.
And Pentecost represents the acceptance of God's spirit, which incorporated into our own human spirit gives us the strength and guidance we need to refrain from further sin.
The initial symbolism of the Spring holy days has been fulfilled, but they still have meaning in the present time.
The Fall feasts:
Again, these four holidays have a greater symbolic meaning, one that would not become apparent until much later.
Trumpets (Lev 23:24)
Each year, the 1st day of 7th month was the first Fall festival day, to be treated as a sabbath and known as the Feast of Trumpets.
Traditionally trumpets were used to signal danger or to precede great announcements.
The Feast of Trumpets symbolizes Christ's return to Earth to put an end to the total mess mankind has made of things.
The armies of the beast and those of its enemies will all be destroyed.
Over the centuries, a small number of people accepted God's teaching lived lives following Jesus's example, and developed God-like character.
Both those that have died and those that are still alive at the time of his return will immediately be reborn as immortal spirit beings.
Those members of this first resurrection (perhaps only hundreds of thousands or only a few million) will now teach and rule on Earth in the Kingdom of God.
Atonement (Lev 23:27)
The 10th day was to be the most sacred holiday of the year and known as the Day of Atonement (Yom Kippur).
Not only was no work to be done, as on sabbaths and other high holidays, but the people were to fast for a night and a day.
It was a time to review one's mistakes over the past year and to contemplate one's relationship with God.
The priests were to take two goats, one was assigned all the sins of the nation and led into the desert, while the other was sacrificed as payment for those sins.
Thus sin was symbolically removed from the nation and past sins paid for.
People were figuratively cleansed of their sins allowing them to once again be at one with God.
The Day of Atonement symbolizes the binding of Satan and his demons (the goat sent into the wilderness).
For the next thousand years the surviving members of mankind will be able to live free of the devil's influence and will be directly instructed and ruled by God's new children.
We will be at one with God and his way.
Tabernacles (Lev 23:34, 36)
The Feast of Tabernacles lasted a week, starting on the 15th, and was set aside to celebrate the Autumn harvest.
It was a time to commemorate the good things that God had provided.
It was also a time to remember the time that the Israelites had spent living in temporary dwellings after leaving Egypt.
During this time families would typically travel to a great centre such as Jerusalem and spend the week away from home, enjoying themselves and not working (much like a modern summer vacation).
Families were expected to set aside 10% of their annual income for these celebrations.
Even those people that could not get away from home would sleep in temporary structures outside their homes.
The Feast of Tabernacles symbolizes Jesus's temporary return to the Earth, teaching and living among mankind.
The world will be managed and governed correctly, with peace and prosperity guaranteed.
And God will make this his spiritual Fall harvest.
During this millennium, most people will understand and accept God's way of life, just as the relatively few members of God's church had done during the preceding centuries.
The Last Great Day (Lev 23:39, John 7:37)
The day following that week was another high holiday, the Last Great Day, when people would assemble for readings of the Torah to ensure that they knew God's laws.
The Last Great Day symbolizes what happens at the end of the millennium.
In a second general resurrection, all mankind that had not had a chance to know God's way will be resurrected.
In the Kingdom they will have the chance to live and learn.
After most of the last generation of mankind has been reborn as immortal spirits there will be one last resurrection, a resurrection of all that have died after rejecting God's way of life.
Those, and the remaining people still living on Earth, and indeed all the surface of the Earth itself will then be totally destroyed in a fiery blast.
Nothing will be left but ashes.
Even memory of them will be gone.
The Earth will then be rebuilt, the Father himself will come to Earth, and his children will spread to new worlds throughout the universe.