I’ve asked these questions to Catholics tens of times, and not once has anyone gotten all answers right. All Catholics are “bound” by the Church to participate in mass every Sunday and “other holy days”. Unless excused for a serious reason, “those who deliberately fail in this obligation commit a grave sin”, and “Grave (mortal) sin deprives us of communion with God, and therefore makes us incapable of eternal life” . In other words, missing a Sunday or holy day immediately puts a person at risk of going straight to hell if they die before confession… do not pass go, do not collect $200.
OK, trivia, here we go. No cheating!
Question #1: What are the “holy days” established by the church for obligatory attendance in addition to Sundays?
Hint: there’s only three of them.
Hint: One is Christmas.
Another is New Year’s Day… What is the third?
Answer: December 8, the Day of The Immaculate Conception.
Question #2: What is celebrated on such an important day that would prompt the Church to declare that any human that knows of it and fails to attend risks spending eternity in hell?
The day Christ first appeared in the Virgin Mary’s womb? Nope.
The Virgin Mary’s birthday? Try again.
Answer: It is the day the Virgin Mary was conceived (the moment where she transformed from sperm+egg to embryo). And why is this event so important? Because she is the first human since Eve to be conceived without original sin.
There’s nothing in the scriptures that mentions any of this, and the belief was questioned and disputed for a long time until Pope Pious IX declared in 1854 that it is true, mainly citing “Sensus Fidelium” (sense of the faithful), which basically means: because it feels right. The way a priest explained the Pope’s rationale to me was: “Is it convenient that she was conceived free from sin? Yes, then it is so”.
 Catechism of the Catholic church, Art 1472