The importance of the spring equinox could not be understated, given the importance of agriculture. For the early Christian church, the time of the Passover was seen as the most important period in the Christian calendar. The importance of Passover, falling near the time of the pagan spring celebrations, may have influenced the Church to decide in 325AD, in the Council of Nicaea, to have Easter fall on the Sunday following the first full moon after the spring equinox. In effect, this puts it near but not exactly at the same time as the pagan rituals. Furthermore, the Church made this celebration a moveable holiday and not fixed. This gives it a range between March 25-April 25 each year, depending on the particular year.
As the church developed the holiday and Christianity spread in the early Medieval period in Europe, particularly in Northern Europe, past pagan traditions of using eggs and rabbits as symbols of fertility and celebration of new life, the Christian church began to use these symbols to reflect them on Jesus as the life giver. In essence, this retained many of the symbols while reorienting the meaning. Initially, it is likely the celebration may have not changed very much in the 4th and 5th century CE, but as the Church's influence spread the holiday began to develop its own new symbols and traditions.The fire used in Mass, for instance, may have connections to worshiping of the sun but in Christian traditions the meaning and importance of the sun was transformed as symbols for the importance of Christ giving light and salvation.