Gee your church hasn’t explained this basic concept to you?
I suggest you find a new church. The Catholic Church is a good one.
In short the OT law and observances were a showdow of the fulfillment of the new covenant fulfillment Jesus Christ and his Church.
For example we catholics changed our observance of the Sabbath from Saturday to Sunday because Sunday was deemed the Lord’s day , his day of resurrection, the day of new covenant fulfillment. Also to catholics baptism replaced circumcision as the rite to join the people of God. The communion meal or eucharist is the fulfillment of the Passover meal, Jesus is the lamb of God who forever takes away the sins of the world he becomes our paschal lamb also it becomes a source for our spiritual meal that was forshadowed when God gave manna to the Jews in the dessert now he gives us Jesus Body, Blood, Soul and divinity in the Eucharistic meal. In many ways cathoicism connects the covenants more explicitly than evangelical Christianity. In Catholicism the new testament fulfillment is always greater than the old testament type. This is not the case in evangelical Christianity.
Thus to celebrate a mere type and lesser fullfillment lets say Passover would be nonsense when you could fulfill that type a greater anti-type with the celebration of the Lord’s supper with Jesus being the paschal lamb we consume in the eucharistic meal not a slaughtered animal. The highest holy day of this fullfillment I am sure you would recognize is Easter the culmination of holy days on our liturgical calender. Like Judaism Catholicism has holy days in her own tradtion of which Chistmas and Easter as most recognizeable. The church calender is a buid up to these holy days. In our view these holyd days represent signficant fullfillments in the history of the chuch. We for example celebrate the birthday of the church Pentacost. Build up Christmas with the season of advent. build up to Easter Lent starting with Ash Wednesday ending with the Triidium of Holy Thursday, Good Friday and Easter.