The Hebrew word mentioned in the Leviticus 23 chapter translated to Polish as a feast- literally means the appointed time. So, the feasts of Jehovah are the times appointed by Him to meet with His people.
It is important to notice that these are not Jewish feasts, but the feasts of Jehovah. God gave them to Israel so that it will teach other nations how to obserthem. However they belong to the heritage of all children of God, both coming from the Jews and also from the Gentiles. The Jews have many other feasts, which are not moedim-the times appointed by Jehovah, such as Hanukkah, Purim. These are their national feasts, like the Independence Day on 11 November in Poland.
Jesus Christ- the Second Moses, while living on the earth observed the feasts of Jehovah. They were kept by the whole New Testament assembly of Israel too, not only by the Jewish believers but also by converted from the Gentiles.
They were changed in the IV century by the politician, the emperor Constantine, to the feast known today as Christian but having their root in paganism.
The feasts of Jehovah are the literal calendar. God uses it to His own things, fulfilment of His prophecies. The most important events in the history of Israel took place in times appointed by Jehovah- the sacrifice of Jesus- the Passover, the birth of Jesus- the Feast of the Tents, filling the believers with the Holy Spirit, etc.
If God asks us to meet with Him in that time, making it in another time, by our own way, will be an expression of rebellion, arrogance and ignorance. Observing the Feasts of Jehovah we show Him our respect, obedience and love.