The feast of the Immaculate Conception is my favorite Marian feast--and not only because it's my birthday. (I'm turning 50 today--tempus fugit!)
It's a great feast because Mary, the sinless one, is a model of what we will one day be. She was preserved from original sin; we were born with it. She never sinned during her life; we sin all too often. But Mary's freedom from sin makes her sympathetic to us who are trapped in its web. She offers us the hope that Jesus brought in redeeming us from sin and death.