Since the dawn of mankind, as archeologists are discovering, humans have found reasons to celebrate certain events, or to mark certain seasonal changes. In recent years it has once agin become fashionable to re-invent, or in the words of some, to reclaim, the major Christian festivals, usually in the name of some 'original' religion, but often just for the sake of making it more 'inclusive' and secular. In recent days I have read several treatises on how Christmas was "stolen" from Nordic pagans, or some other group or people, so perhaps it is time to look afresh at why Christianity adopted certain dates in order to mark key events or beliefs.
The one festival we can be reasonably certain of is the Easter festival. Since St John tells us the trial, condemnation, crucifixion and resurrection all occurred "on the eve of the Passover". It made sense therefore to the early Christians to mark the Jewish Passover by commemorating the death and resurrection of Jesus, the Christ. The problem is, of course, that it isn't a "fixed" feast in the sense that it may occur anywhere between March 21 and April 25. The Jewish formula, the Orthodox Church and the Western Churches all have slight variations on the way they determine it (Rome, responsible for the way we calculate Easter in the West, changed the formula at least three times between 325 AD and 700 AD) which means, of course, that while Easter and Passover often do coincide, sometimes they don't. It also means that the Western Easter and the Orthodox Easter often don't coincide either. Personally I don't believe this makes one jot of difference - the key is that we remember what Easter is all about - the fact that Christ died on the cross (forensic analysis of the accounts makes that clear!), and rose from the tomb in a renewed 'body' on the third morning. It is this 'resurrection' which is key to the entire Christian Faith. That is why Easter is so important to all Christians.
Having dealt with the reason Easter is 'moveable' and not 'fixed' let us look at some of the other major festivals. The obvious 'biggy' is Christmas, set as the 25th December by the early church fathers. It certainly didn't take its present form however until quite late, and it isn't, and never was, intended to mark the actual day of Christ's birth. Again, examination of the gospel accounts more or less tell us it was very likely in summer. Shepherds and sheep aren't out on the hills in winter, even in the Holy Land. So why did the early fathers choose this date?
The answer is simple. Many of their converts already celebrated the Saturnalia between 21 December and 1 January. Primarily the Saturnalia is a fertility festival associated with a fairly wide range of 'fertility rites' including public copulation as an "offering" to some local deity. Under Romano-Greek practice Bacchus, Satyrs, Pan and several other 'fertility' figures featured in the celebration which marked, as you've guessed, the gradual move out of the winter and into the Spring rebirth of fields, forests and animals. Theologically it does not take a great deal of effort to link the Birth of the New Adam - Jesus Christ - to a renewal of all life, and the renewal through the 'New Adam' of all humanity as well. As St Paul did with the Athenian altar to "the Unknown God", all that really happened was that the Christian converts found a new reason to celebrate the Saturnalia, though we may be sure they didn't continue with some of the more carnal excesses!
Over time, and with the spread of Christianity, the Winter Solstice celebrations in Northern Lands were 'converted' to the Christian message, again, largely because converts already kept these celebrations, and simply were given a new interpretation of them. Thus, Yule logs, Christmas trees, decorations and lights entered the celebrations. As the faith spread, each 'new' group of converts found ways to celebrate their new faith within the framework of the celebrations of their 'old' faith. Gradually these have been 'standardised' by the wider church, but still retain a lot of local and regional variations.
This is how some of the Imbolc customs have become associated with Easter, and how some of the Samhain customs associated with the Festival of All Saints. Imbolc, a festival celebrated by the "Celtic" tribes of Northern Europe and the British Isles doesn't coincide with Easter as some think and actually falls in February. It was originally a celebration of the increase of flocks and herds, since it marks the birthing of lambs, wild boar and deer - and the original cattle. Likewise Samhain doesn't necessarily fall on what has become "Halloween" (a corruption of "All Hallows Eve", the evening preceding All Saints) which is, again, an amalgam of several similar pagan festivals celebrating Harvest, honouring the dead, and invoking local 'gods' and spirits to tide one over the winter. By switching the focus to a commemoration of the "Saints" who set us an example, and adding a commemoration of all those departed this life, the early church created a moment for reflection on life and thanksgiving following the intense labour of the summer, the harvest and the preparations for the long cold, lean months of the winter.
There is a logic to the progression of the festivals and feasts the Church adopted and it has worked very well, by and large. Of course one could argue that some of the original focus has been lost - no one would, I think, disagree that since the 1850s at least, Christmas has been gradually turned into a shopping extravaganza. One could also be excused for wondering what some folk think they are really celebrating in their church/family traditions, and I will confess to being a little concerned at some practices.
What has always intrigued me, and which I have still not been able to find an answer to, is the question of the reason why so many very diverse cultures chose to mark similar festivals at very similar periods. Christianity has adopted largely northern and western European festivals, but one can find similar festivals and seasons in many other cultures. Some, obviously, are related to the 'cycle' of the agricultural year, while others are not. Answers on a postcard please ...