Occult Holidays or God's Holy Days - Which?

What is the true origin of today's widely accepted holidays, Halloween, Christmas, Easter, etc., and how did such days come to be embraced as "Christian" dogma? Likewise, what happened to the biblical festivals God gave to ancient Israel and which were observed by the original, true apostolic Church? Through the centuries, how and why were God's holy days recklessly cast aside as obsolete by orthodox "Christian" authorities?