Easter was always the principal holy day in the Christian calendar--even though, in recent years, it has gotten elbowed to the side in the US by Christmas and even Hallowe'en. It is a so-called "movable feast" because it is celebrated on a different day every year, moving around in late March and April.
This is because its celebration is based on Passover, which, like all Jewish holidays, is based on phases of the moon. According to the Gospels, Jesus had come to Jerusalem to celebrate Passover when he was crucified. Both holidays are supposed to be the first sabbath after the first full moon after the equinox. Because the Jewish sabbath (Saturday) and Sunday are different days of the week, they do not always occur together in the modern calendar, and exactly when the moon is considered "full" is open to interpretation. A major dispute broke out in England in the seventh century between the Irish and the Roman versions. In 2015, Passover and western Easter are on the same schedule, but Greek Orthodox Easter comes a week later. Medieval monks made elaborate tables to calculate when Easter would fall each year.
Because many medieval calendars started the New Year with Easter, there could be years with two Aprils, if Easter was early one year and late the next. Fortunately what year it was did not figure as significantly for medieval people as it does for us.
Medieval theologians all agreed that the Resurrection, the first Easter, occurred on March 27, because Jesus "of course" would have died on March 25. This is because important people died on the anniversary of their conception, and they concluded that Mary would have conceived on March 25 because Jesus was born on December 25. It all made sense.
Easter was by far the most important Christian feast day. Coming after 40 days of Lent, where people really did try to give up at least some normal pleasures and follow a sparse diet (for one thing, most of the food stored away at harvest time was exhausted), Easter really was a feast day as well as a holy day.
As a holy day, it was the day that all Christians were supposed to go to church, even if they didn't the rest of the year. (A lot of modern-day Christians seem to follow this same directive.) A monastery might have a little play, in which monks wearing white robes represented the women coming to the tomb at dawn of the first Easter, finding it empty, and learning, "He is risen."
That women, rather than men, were the first to learn of the Resurrection and the first to see the risen Christ was one of the details that made medieval theologians agree that women were equal with men in the eyes of God, even if not in secular law. (Click here for more on medieval women's position.)
Most societies have some sort of spring festival, celebrating renewal and rebirth, but medieval Easter celebrations kept the focus not on generic renewal but specifically on the Christian version. Modern Easter, with its rabbits, eggs, and chocolates, to say nothing of new spring outfits, focuses on symbols that medieval theologians would have found deeply troubling.
For my further thoughts on Easter and Christmas, see my long essay, available as an ebook from Amazon and other e-tailers.