Well, I will cover the ancient feast day of Samhain instead of the modern pagan celebration:
There are many accounts of Samhain in both he Medieval sagas and historical records, dating back to at least the ninth century (800's). Most of these accounts are fairly neutral, with a few Christian flourishes thrown in here and there, but a few are very obviously biased accounts that were openly meant to paint the Irish pagans in a negative light.
The stories of Samhain, which speak of events that happened centuries before they were written down, mention a feast held for the warriors and chieftains of Ireland, epic battles between various races that have supposedly inhabited Ireland over the centuries, divination by the Druids, and interactions between mortals and the world of the sidhe (this was the fairy realm which was very different from both the modern depiction of fairies and the Christian world of the dead). The accounts clearly mark the day as being November 1 (the same as All saints Day) although this was according to the calendar that was imported with the coming of the Christians. In the 19th century, according to my study of Gaelic dictionaries, Samhain evolved from meaning November 1st to denoting the entire month of November (the old word for November was Naoimhi). As is mentioned above, the Irish Gaelic term for Halloween is oiche shamhna (ee-huh how-nuh) and means the 'Eve of November'.
You only asked about Samhain, however: All Saints Eve and All Souls Eve are Roman Catholic holidays that developed separately from Samhain - in fact feasts to honor the saints existed before the church was aware of Samhain. All Hallows Eve means All Saints Eve, so-called because it is the night before All Saints Day, November 1st. When All Hallows came to Ireland, and subsequently Scotland, it combined with both Samhain and folk traditions that already existed there and completely changed, eventually being named "Hallowe'en". It no longer resembled the Catholic feast day, but instead became an eve for supernatural beings, divination games, and parties. (Although it did retain its Christian connection to death, as, contrary to popular belief, there is no evidence that Samhain had anything to do with the dead - that is a modern pagan function of the celebration). When the Irish and Scottish brought Halloween to America it was changed even more, and now it is considered an American holiday.
All Saints Day is still a public holiday in many European countries, with no relation to any Scottish or Irish traditions, but not in Ireland. Ireland observes the secular Halloween with a bank holiday in October.
Midsummer Eve was celebrated by pagans because it is the summer solstice and for Catholics it is Saint John's Eve. Like most holidays of the past, St. John's Eve had superstitions and supernatural activity associated with it.
EDIT: I should have mentioned:
Samhain was also the opening of the winter half of the year, with May 1 being the opening of the summer half. This is documented in history. The contemporaneous historical accounts are a little different from the sagas (which probably contain some actual history, as they claim to) in that it is clear that Samhain was still a day important enough to be acknowledged, but how much of its ancient observance remained is a matter of debate.
The celebration of the solstices was common to many pre-Christian cultures. This is well-know to archaeologists.