Jewish feasts, why are they related to Christianity?
They were pre-events to the true sacrifice, later accomplished by Jesus' life and crucifixion for our redemption. Palm Sunday's entrance into Jerusalem right behind Jewish priests with the unspotted lamb from Bethlehem as a sacrifice to the Temple is not just a coincidence.
At the Last Supper, where Jesus delays the last Passover cup until His crucifixion, by saying, "I am thirsty" on the Cross, and drinking vinegar as the last cup, He is finalising His sacrifice as the Passover lamb (studied by Scott Hahn). This is no coincidence either. Saint Thomas Aquinas said in 1250,
Because the mystery of Christ, which was foreshadowed by these ceremonial precepts ... Now the sacraments of the Old Law foretold the coming of Christ. Consequently they did not signify Christ so clearly as the sacraments of the New Law, which flow from Christ Himself, and have a certain likeness to Him.
The Pentecost and Holy Spirit (Shavuot - Feast of Firstfruits) after Jesus's Ascension are all Jewish traditions, accomplished by Jesus' life, death, and resurrection.
Book: Understanding Jesus: Cultural Insights into the Words and Deeds of Christ by Amaral, Joe
Book: A Father Who Keeps His Promises: God's Covenant Love in Scripture by Hahn, Scott
Order before education. The Old Testament taught order, through laws and rituals, in preparation for the Messiah. The New Testament now teaches a soul how to achieve his or her own salvation.
Did Jesus Christ make an appearance 1200 years before He was born? Yes, to Joshua, in command of Israel after Moses. Let's read from the Old Testament, Joshua 5:13-15,
And it came to pass, when Joshua was by Jericho, that he lifted up his eyes and looked, and, behold, there stood a man over against him with his sword drawn in his hand: and Joshua went unto him, and said unto him, Art thou for us, or for our adversaries? And he said, No, but I am the Prince of the host of the LORD; now I am come. And Joshua fell on his face to the earth, and did worship, and said unto him, What saith my Lord unto his servant? And the captain of the Lord's host said unto Joshua, Loose thy shoe from off thy foot; for the place whereon thou standest is holy. And Joshua did so.
Close examination of the passage with a comparison with the rest of Scripture identifies him as the Son of God; and Joshua, as the study of the Pentateuch demonstrates, had a revelation which enabled him to clearly identify him as the Divine Savior, Mediator, and King. More Old Testament references at the end of this site.
Book: The Holy Bible: Old Testament: Douay-Rheims Version by Catholic Way Publishing
Brant Pitre's study guide is a must when reading the Old Testament. For example, modern time critics suggest some Old Testament prophets accepted the abuse of women in society; let Pitre answer,
Contemporary secular society no longer considers adultery a crime, but in fact glamorizes it in literature and cinema and also approves of maternal infanticide under the guise of abortion, it is somewhat unsurprising that the figure of adulterous, child-murdering Jerusalem should attract more sympathy among contemporary commentators than it did among Jews and Christians in antiquity.
Book: A Catholic Introduction to the Bible: The Old Testament by Brant Pitre
Robert Bellarmine (1542-1621), one of only two Jesuit Doctors of the Church, commented on each of the Old Testament Psalms and their relationship to the New Testament,
Book: A Commentary on the Book of Psalms by St. Robert Bellarmine
If God is unchangeable, why do we have different commands throughout human history? Saint Thomas Aquinas in 1250 answered,
Just as the head of the house is not proved to have a changeable mind, through issuing various commands to his household at various seasons, ordering things differently in winter and summer; so it does not follow that there is any change in God, because He instituted sacraments of one kind after Christ's coming, and of another kind at the time of the Law. because the latter were suitable as foreshadowing grace; the former as signifying the presence of grace.
First-century Jewish society was as fractured as our Christian society today,
Pharisee in the Hebrew signifies ' divided ;' because choosing the justification of traditions and observances they were ' divided' or 'separated' from the people by this righteousness. Sadducee in the Hebrew means 'just ;' for these laid claim to be what they were not...they only received the Pentateuch, and rejected the Prophets.
Book: Catena Aurea by Saint Thomas Aquinas
First century "Pharisees" are now modern-day radical Catholics, opposing Pope Francis. "Sadducees" then, protestant Christians now (accepting the New Testament while rejecting the saints). "Herodians" then, modern Christian cults now. The respected "Essenes" sect then, Catholic cloistered religious now. Faithful Jews then, Christian laity now. Idolatrous religions then, non-Christian religions now. But God loved His true Jewish prophets then, and He is showing the same love today to His true Catholic saints.