It wasn't by coincidence that the Church chose to celebrate All Saints' Day on November 1, or Candlemas on February 2.

The Celtic pagan holidays were taken over and Christianized, and the placement of the Christian holidays had nothing to do with celebrating an event at the anniversary of its actual occurrence (except Easter, which was tied to the Jewish Passover and so happened at the right time of the year without fail). It is likely that Jesus was born in the Spring if the Gospels are to be believed; in the Near East shepherds would only watch their flocks by night in the early Spring when the lambs were born.

All Saints' Day, a day venerating a number of people, didn't happen to fall on Samhain, the feast for the dead. It was specifically chosen for that day, just like the Presentation of Jesus at the Temple was called Candlemas in honor of the traditions of Imbolc. Beltaine was never really subsumed by a Christian holiday and everyone just kept celebrating May Day. Lughnasadh was turned into Lammas-Day, the day on which the harvest was blessed.

Each of those days was halfway between an equinox and a solstice, and based on old solar calendars. That would have nothing to do with the roots of Christianity, since Judaism followed a lunar calendar, and no particular significance would attach to solar calendar breaking points.