Babylon is fallen: blending paganism with Christianity

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Credits to pastor Craig, my brother in Christ. His YouTube channel and website is SDRMinistries.org.

Xmas, easter and Sunday observance are considered as important feast days or high days in today’s mainstream Christianity including Roman Catholicism, Protestantism and even, some SDA churches.
Where are they mentioned in the Bible or commanded by the Creator God? Can we blend feast days from paganism into Christanity? Or should we be separate from what does not come from God (2 Corinthians 6;17)?