As Constantine was intent on converting the Roman Empire to Christianity, he understood that the people would not willingly surrender all of their pagan feasts. To require them to do so, so suddenly and so abruptly, would risk a revolt and he could/would not chance it. So he merged them into the Christian teaching, re-designating their purposes so as not to conflict with his newly found faith.
Because of this history, there are some who will not celebrate these days because they feel that to do so, constitutes them celebrating a day rooted in idolatry. In Paul's days of traveling the countryside preaching the Gospel. (as a Roman citizen), he explained it well in this passage from I Corinthians 10:
27 If any of them that believe not bid you to a feast, and ye be disposed to go; whatsoever is set before you, eat, asking no question for conscience sake.
28 But if any man say unto you, this is offered in sacrifice unto idols, eat not for his sake that shewed it, and for conscience sake: for the earth is the Lord's, and the fulness thereof:
29 Conscience, I say, not thine own, but of the other: for why is my liberty judged of another man's conscience?
30 For if I by grace be a partaker, why am I evil spoken of for that for which I give thanks?
31 Whether therefore ye eat, or drink, or whatsoever ye do, do all to the glory of God.
32 Give none offence, neither to the Jews, nor to the Gentiles, nor to the church of God:
33 Even as I please all men in all things, not seeking mine own profit, but the profit of many, that they may be saved.
Paul traveled a lot and naturally, he got hungry. When in town, often there would be some kind of a feast taking place in an idol's temple. Paul was hungry and they were serving food. He thought nothing of buying some food, giving thanks to God for it, and eating it....sometimes in the very temple with the people who did not share his beliefs.
To Paul, the idol was nothing. He knew who was responsible for all that he received and never once did it cross his mind to care if the cow or lamb was sacrificed to an inanimate object made out of a stone carving. It did not affect his conscience in that way. All he knew was he needed food to continue in his mission and God was providing it.
So today, we know that although these holidays were originally conceived in pagan cultures, we can now feel no guilt or shame in taking time out on these days to celebrate them, for our own reasons. We do not honor those stone carvings from those days, we honor a risen saviour, who sits at the right hand of His Father, the entity we know as God. He was born into a world for the specific purpose of sacrificing His earthly life, for all of us. After His earthly anguish was conquered, He willingly did it, knowing that God was able to and would raise Him back up on the third day, as it was prophesied many times before.
Happy Easter to all.