Welcome to the weirdness that is the current US calendar for the Catholic Church. Yesterday was December 8th, the Solemnity of the Immaculate Conception of the Blessed Virgin Mary.
Under this title, Our Lady is the patroness of the United States, so here in the U.S., we usually mark the feast of the Immaculate Conception as a Holy Day of Obligation. However, when December 8th falls on a Sunday (like yesterday), the feast is transferred to the next day (i.e., today).
And in the U.S., when a feast that is normally marked as a Holy Day of Obligation falls (or is celebrated) on a Saturday or a Monday, the "obligation" to attend Mass is removed. Why? Because, (and this is the honest reason), some time ago the US bishops decided that it was too burdensome on the faithful in this country to "oblige" them to attend Mass two days in a row (since Sunday is always a Holy Day of Obligation).
At any rate, St. John's will be offering Mass today at 12:10 pm for the Solemnity of the Immaculate Conception. So, despite what some folks think you might be capable of, you're welcome to burden yourself and attend!
