wheel of the year
When the Pagans were dwelling in the Medieval times the only way to calculate the months of the year was by recognizing the seasons. Therefore the calendar we use today is not at all like what the Pagans used in their day.
Samhain is the modern day Halloween. Today we celebrate by dressing up and going Trick or Treat. In Pagan tradition Samhain was the day that actually ended summer and kicked of the Celts new year. The end of harvest and the start of winter that was pretty brutal in those days.