It is what is called a holy day of obligation, when Catholics are expected to go to mass. The Church teaches that the Virgin Mary was born without sin. This special feast has been celebrated since the 7th century.
This is a huge stumbling block for Protestants, mainly because of the scripture passage in Romans 3:23 that states "all have sinned and fall short of the glory of God". The word "all" in Greek, pas, doesn't necessarily "every single one without exception". For example, St. Paul also writes in Romans 11:26 that "all Israel will be saved."One would suspect that not every single Jewish person will be saved.