Why don’t we follow the Jewish feast days? And why does the Church have different feast days for Saints etc.?
I might be able to help with the first part of your question,
Many of the Jewish feasts/holidays are memorials of what God did for the Jewish People.
Passover celebrates God redeeming the Jewish people from slavery to the Egyptians.
Chanukkah celebrates the multiplication of the holy oil in the reconsecration of the temple after the Maccabee revolt.
Purim celebrates the redemption of the Jewish people from the hands of Haman.
The Feast of Booths (Sukkot) is a memorial of when the Jewish people lived in tents as they wandered through the desert.
So these feasts are basically only applicable to them.
(I bolded ‘‘basically’’ because Passover has far-reaching implications)
We can look at these feasts and see Jesus:
Christ is our Passover Lamb; The Feast if Booths–God came to dwell among us; Chanukkah–Christ is the light of the world, etc., but we really don’t celebrates these feasts because they were given to the Jewish people so they might always remember what God had done for them. For us as Christians, we believe that God has fulfilled them all in Christ