Today (December 8th) is the feast of the Immaculate Conception. In the US, this is a holy day of obligation (meaning we need to attend Mass just like on a Sunday), so don't forget to get there!
This is the day we celebrate the conception of Mary without original sin. No, this is not the conception of Jesus within Mary, but of Mary herself. If you have any questions about why Catholics believe Mary was conceived without original sin, check out these two posts:
Have a great feast day, everyone!