In the Christian liturgical year, three kinds of commemorations exist. The first kind are obvious: they are the anniversaries of important events in the faith: typically the anniversaries of martyrdoms or anniversaries of the births or miracles of unmartyred saints, or of the consecrations of important churches, and so on.
The latter two kinds commemorate the story of salvation, told and re-told on an annual basis. In order to tell the story, the events of Jesus' life are placed in chronological order and then slotted into the year based on the dates of Easter and Christmas. They aren't anniversaries of the events in Jesus life, obviously, because if they were they wouldn't fall in neat chronological order. Remember, we're trying to fit 33 years of life into a one-year dramatic retelling.
In almost all cultures, save English-speaking ones -- and even in English-speaking cultures among the Eastern Orthodox -- the celebration of Christ's resurrection takes it's name from Passover, or Pesach, which is when the crucifixion and ressurection occurred. It's calculated based on a lunar rhythm: the first Sunday following the first full moon following the vernal equinox. Several other Christian holidays: Ascension day, Pentecost, the forty days of Lent, are calculated off the date of Easter. They move back and forth with the phase of the moon, so they are called "movable feasts".
The third kind of holidays fall on the same day every year and are called "immovable feasts". Eight of them in particular fall on the equinoxes and solstices, and on the half-way points between them. They're intriguing, in that their placement seems in many cases to be rather arbitrary. From folk-customs associated with these nominally Christian celebrations, social archeo-historians have reconstructed what they believe were originally pagan holidays, preserved over the period when state-sponsored Christianity overrode the pre-Christian cultures of those lands. Records of how those Christian holidays came about are of course quite scanty, but they do seem to be marked by syncretism. On the other hand, Christian theology does accept that the God who came to earth as Jesus the Christ, also set the stars in the heavens and ordered the seed-time and harvest, so it is as logical that ancient agrarian Christians should celebrate the turning of the seasons as that ancient agrarian Pagans should do so.
The English word "Easter" is simultaneously one of the best, and worst, indicators that syncretism was involved in establishing the holidays on the quarter- and cross-quarter days. "Easter" actually comes from the name of the pagan spring-goddess, Oestra (and other variant forms). But Oestra's day is believed to have been the vernal equinox, not the lunar feast of Pesach. The equivalent Christian celebration is "Lady-Day", or the feast of the Annunciation to the Blessed Virgin, which is conveniently located exactly nine months before Christmas. If the Christian holidays were taken from the Pagan ones, you would expect Lady Day to have received the name "Easter". But if they did not come from the Pagan feasts, then you wouldn't expect to find the name "Easter" used at all.