Question regarding Feast Days in Penitent Seasons

Okay, so we're in Advent, which is a penitent season. But there are a whole bunch of feast days, obviously, including last Monday's Feast of the Immaculate Conception. I understand that not all of the feast days are obligatory -- some folks feasted on St. Nicholas Day, some didn't. Some are feasting today for Our Lady of Guadelupe, some aren't.

So how does this work if a feast day comes up that you want to celebrate (patron saint, say) during a penitent season? Do you observe penitent practices except for the evening meal? Observe the feast all day and resume the penitent practices the next day?

--Sparki