Tomorrow, obviously in the United States at least. The Bishops have decided that since The Feast of the Assumption of the Blessed Virgin Mother falls on a Monday it will not be celebrated as a Holy Day of Obligation in the United States. Most parishes do not have Mass on Monday because that is the pastor's day off, so in most cases there won't even be a regular daily Mass.
Luckily our pastor was looking out for us. He was going to be away on the World Youth Day Pilgrimage, but he thought it was important enough to have Mass on that day, so we are actually having 2 priests come, one to say a daytime Mass and another to say an evening Mass. We listed them as celebrations of the Feast without mentioning that it wasn't a Holy Day. What they don't know won't hurt them right.
If the congregation comes to mass thinking they are obliged to do so nothing is harmed. If they come to mass because they want to celebrate the Feast all the better.
My thought on the matter is if you have the mass they will come.