The Feast of the Immaculate Conception will be on a Monday this year. If I go to the evening Mass on Sunday, will that fulfill my obligation for Monday?
It can. Any eucharistic liturgy celebrated in a Catholic rite fulfills the Mass obligation for a Sunday or holy day, and so the Mass you attend for this coming holy day need not be specifically for the Immaculate Conception. But you must attend two Masses, one for Sunday and one for the holy day. You cannot attend one Mass for both obligations. So, for example, you could attend a Mass on Saturday evening and Sunday evening, a daytime Mass and evening Mass on Sunday, a Saturday evening Mass and a Mass on Monday, or a Mass on Sunday and a Mass on Monday.