Christians have been celebrating the Annunciation on March 25th since the 2nd century. Christmas celebrations developed more gradually, but the December 25th date has less to do with celebrations of the Winter Solstice and more to do with the fact that December 25th is nine months after March 25th. It’s not actually our fault that Christmas is more fun than whatever they were doing on December 21st, and I also doubt that anyone was becoming a Christian only for that reason.
All Saints’ Day has traveled all over the calendar; it is only relatively recently that we started celebrating it on November 1st. I have no idea about the history of Samhain; I only started hearing of it when I was in my teens, and I don’t know where it came from.
There is no evidence Samhain was a religious holiday to the Celts. We have NO contemporaneous textual evidence on pre-Christian Celtic celebrations, BTW, as they were a pre-literate people without a written language. What knowledge we have of Samhain is from medieval times, as recorded by monks. The word itself is the name of a month (November) not the name of a Celtic “God of the Dead,”, so this was just a way of referring to the 1st day of November. It also had little to do with the harvest (long past by this time of the year) but was associated with the movement of cattle from the highlands in the Celtic countries. This was a time with the tribes and clans would meet on the plains, slaughter cattle, have trading fairs, settle political disputes, and so on. It had no religious significance, and contrary to popular folklore, had no connection to a celebration of the dead (as All Saint’s Day and All Soul’s Day do.) The confusion comes from Sir James Frazer and other Victorian folklorists, who insisted that all Catholic religious festivals must have pagan roots, so All Soul’s Day and All Saint’s Day
must have a pre-Christian antecedent. As jmcrae noted, there were several dates that were used by the early Church in different regions to say a mass for the Martyrs and those in Purgatory. The November 1 date originated in Germany, so it would be hard to argue that it was somehow borrowed from a Celtic pagan festival.
It’s interesting that one of the most popular All Hallow’s Eve celebrations throughout Europe in the middle ages was ringing the church bells all night long, from dusk to dawn, using shifts of the town’s men and boys (it was very popular and not hard to find volunteers - who wouldn’t want to make noise all night?) (There may have been some ale involved, I would guess). The idea was to make enough noise that the souls of the departed in Purgatory would be able to hear the sound from all over Christendom, and would take heart from knowing that we remember them and are praying for them. In England under Elizabeth, this was one of the last of the Catholic customs to be suppressed, as local Protestants complained that locals would still sneak in to the churches to do it. Praying for our dead is such an natural belief that one can still find rural spots on maps with names like “Purgatory Valley” or “Purgatory Hill” where the faithful would gather to pray for their dead in secrecy.
… but what ever it takes to give glory to our Lord Jesus Christ!